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THE  EVOLUTION  OF 
ANIMAL  INTELLIGENCE 


BY 


S.  J.  I  HOLMES,  Ph.  D. 

ASSISTAa»-«lOFESSOB  OF  ZOOLOGT  IN 
THE  UXIVEBSITY  OF  WISCONSIN. 


» 

^^ 

ones 

OT 

j» 

H7Se. 

1911 

NEW    YORK 

HENRY  HOLT  AND  COMPANY 

1911 

17 

5425 

GOPTKIOHT,   1911 
BT 

HENRY  HOLT  AND  COMPANY 


CAMKLOT    PRKSS,    444-46    PEARL   STREET,    NEW    YORK 


PREFACE 

The  investigations  of  recent  years  have  added  so  much  to 
our  knowledge  of  the  activities  of  animals  that  any  adequate 
account  of  the  whole  field  of  animal  behavior  would  require 
several  volumes.  The  present  work  is  confined  to  certain 
topics  which  bear  upon  the  evolution  of  animal  inteWigence, 
and  our  treatment  of  even  this  part  of  the  subject  has  been 
of  necessity  fragmentary.  It  has  been  our  aim  to  give  a 
fairly  clear  conception  of  the  activities  upon  which  intelli- 
gence is  based,  to  show  how  intelligence  is'related  to  these 
activities,  and  to  sketch  the  general  course  of  the  evolution  of 
intelligence  in  the  animal  kingdom.  No  effort  has  been 
made  to  deal  with  all  the  classes  of  animals  in  which  intelli- 
gence is  manifested,  and  some  groups  which  were  not 
essential  to  the  development  of  our  theme  have  received 
little  attention. 

I  wish  to  express  my  thanks  to  Prof.  L.  J.  Cole  and  Prof. 
H.  B.  Torrey  for  their  helpful  criticisms  of  the  manuscript  of 
this  book  before  it  went  to  press.  To  Prof.  L.  T.  Hobhouse, 
Prof.  E  *  L.  Thorndike  and  Dr.  L.  Witmer  I  am  indebted  for 
permission  to  reproduce  the  figures  which  are  attributed  to 
these  writers  in  the  text,  and  I  am  also  indebted  to  the 
Macmillan  Company  for  kindly  allowing  me  to  reproduce 
figures  15  and  16  from  their  publications.  My  greatest  debt 
is  to  mv  wife  for  helo  and  encouragement  in  many  ways. 

S.  J.  H. 


m 


CONTENTS 


I.     Introduction 1 

II.     Reflex  Acno:, 11 

III.  The  Tropisms 18 

IV.  The  Behavior  of  Protozoa 63 

V.     Instinct 91 

VI.     The  Evolution  of  Instinct »  .   115 

VII.  The  Non-intelligent  Modifications  of  Behavior  .    .    .    139 

VIII.  Pleasure,  Pain,  and  the  Beginnings  of  Intelligence   .   164 

IX.  Priliitive  Types  of  Intelligence  in  Crustaceans  and 

Molluscs 180 

X.     Intelligence  in  Insects 191 

XI.     Intelligence  in  the  Lower  Vertebrates 218 

*   XII.     The  Intelligence  of  Mammals 232 

XIII.     The  Mental  Life  OF  Apes  AND  Monkey.=; 260 

Index 283 


THE  EVOLUTION  OF  ANIMAL 
INTELLIGENCE 


CHAPTER  I 
INTRODUCTION 


"If  the  doctrine  of  Evolution  is  true,  the  inevitable  implication 
is  that  Mind  can  be  understood  only  by  obser\dng  how  Mind  is 
evolved." — Herbert  Spencer,  Principles  of  Psychology. 

*' AUe  instinktiven  Triebe  und  zweckbewussten  Willensausserungen 
dienen  entweder  der  Erhaltung  des  eigen  Lebens  oder  der  Erzeugung 
und  Pflege  der  Nachkommenschaft." — Schneider,  Der  Thierische 
Wille. 

"Si  la^psychologie  humaine  est  legitime,  la  psychologie  compar^e 
Test  aussi,  pour  les  memes  raisons." — Claparede. 

In  the  last  two  decades  the  science  of  animal  psychology 
has  had  an  unprecedented  development.  Previous  to  this 
time  and  subsequent  to  the  appearance  of  the  Origin  of 
Species  students  of  the  animal  mind  were  concerned  in 
bringing  forward  proofs  of  the  similarity  of  the  mental 
processes  of  men  and  animals  and  in  tracing  the  gradual 
unfolding  of  the  mental  powers  in  passing  from  lower  to 
higher  forms.  The  followers  of  Darwin  in  their  efforts  to 
show  that  evolution  is  applicable  in  the  mental  as  well  as  the 
physical  world  were  naturally  prone  to  minimize  the  gulf 
separating  the  human  and  the  animal  mind  which  the  tradi- 
tional psychology  had  formerly  taught  was  a  wide  and  im- 
passible one. 

1 


2  INTRODUCTION 

Hence  we  find  among  writers  of  the  period  mentioned  a 
tendency  to  interpret  the  actions  of  animals  as  the  outcome 
of  highly  developed  psychical  faculties,  and  to  accept  rather 
too  uncritically  stories  indicative  of  great  sagacity  on  the 
part  of  animals  with  the  end  of  showing  that  the  brute 
creation  does  not  stand  so  very  far  below  us  after  all.  The 
works  of  Romanes  on  Mental  Evolution  in  Animals  and 
Animal  Intelligence  are  typical  of  the  period  and  its  tenden- 
cies toward  anthropomorphism,  which  is  now  the  bete  noir 
of  every  dabbler  in  the  subject  of  animal  behavior.  While 
Romanes  has  been  criticized  with  a  certain  amount  of 
justice  for  basing  conclusions  on  anecdotes  from  more  or 
less  questionable  sources  he  has  the  merit  of  bringing  to- 
gether a  large  number  of  facts  regarding  animal  intelHgence 
and  of  giving  an  able  and  lucid  account  of  the  general  course 
of  mental  evolution,  and  there  is  no  doubt  that  had  his 
life  extended  into  the  period  of  more  careful  and  accurate 
experimentation  he  would  have  been  among  the  first  to 
appreciate  and  encourage  the  newer  animal  psychology. 

The  advent  of  experimental  psychology  with  its  various 
appliances  for  observing  and  recording  the  manifestations 
of  mental  phenomena,  the  study  of  the  functions  of  the 
nervous  system  by  the  experimental  methods  of  the  phy- 
siologists, and  the  growing  vogue  of  methods  of  experimental 
analysis  in  biology  in  general  could  not  fail  to  have  a  marked 
influence  on  the  study  of  the  mental  life  of  animals.  After 
the  scientific  world  had  comfortably  settled  down  in  the 
belief  in  continuous  mental  evolution,  and  the  subject  had 
mainly  lost  its  controversial  interest,  attention  became  more 
strongly  directed  to  the  analysis  of  animal  behavior  with  the 
end  of  explaining  why  animals  act  as  they  do.  Then  came 
the  period  of  reflexes  and  tropisms  and  the  analysis  of  com- 
plex behavior  into  simpler  processes,  with  which  we  are  now 


INTRODUCTION  3 

struggling.  Animal  psychology  blossomed  into  an  experi- 
mental science,  proud  of  her  new  growth  and  often  looking 
back  with  something  of  scorn  upon  the  days  of  her  infancy. 
The  position  of  animal  psychology  among  the  sciences  is, 
however,  a  somewhat  peculiar  one.  Concerning  the  con- 
scious life  of  animals  as  distinguished  from  the  objective 
facts  of  behavior — the  life  which  falls  within  the  pecuHar 
province  of  psychology  as  distinguished  from  physiology — 
our  knowledge  rests  upon  an  insecure  foundation.  We  have 
no  means  of  cognizing  directly  the  conscious  states  of  any 
creature  besides  ourselves  and  what  we  know  of  the  psy- 
chology of  our  fellow  human  beings  is  based  upon  what  we 
find  taking  place  in  our  own  minds.  We  infer  conscious- 
ness in  other  beings  because  we  are  conscious  ourselves, 
and  we  judge  of  the  mental  states  in  the  minds  of  others, 
such  as  joy,  sorrow,  anger,  or  fear,  from  certain  physiolog- 
ical manifestations  which  are  like  the  accompanying  mani- 
festation of  these  mental  states  in  ourselves.  With  beings 
much  like  ourselves  our  inferences  may  be  fairly  accurate. 
When  thrown  amid  the  people  of  other  nations  or  races 
our  judgments  are  more  apt  to  be  erroneous.  And  when 
we  try  to  infer  what  goes  on  in  the  mind  of  a  cat  or  a 
dog  the  difficulties  are  very  greatly  increased.  We  may 
feel  con\dnced  that  the  sensations  of  these  animals  and 
even  the  basic  emotions,  such  as  fear,  anger,  etc.,  are 
very  similar  to  our  own,  but  the  difficulty  of  ascertaining 
what  sort  of  intellectual  life  one  of  these  creatm-es  enjoys 
is  evinced  by  the  very  different  interpretations  of  the 
subject  wliich  have  been  made  by  competent  psychologists. 
If  we  try  to  imagine  what  sort  of  psychic  states  are  as- 
sociated with  the  supra-oesophageal  ganglion  of  an  ant  or 
a  crayfish,  analogy  almost  completely  fails  us.  However 
much  we  may  learn  about  the  behavior  of  these  creatures, 


4  INTRODUCTION 

we  could  never  feel  much  confidence  in  any  conclusions  as 
to  the  kind  of  conscious  states  which  make  up  their  mental 
life.  We  may  be  asked,  in  what  way  do  we  know  these 
animals  have  conscious  states  at  all?  May  they  not  be  as 
Descartes  has  considered  all  animals  below  man,  merely 
unconscious  automata?  The  question  throws  us  back  upon 
what  criterion  we  adopt  of  consciousness  and  upon  this 
subject  the  opinions  of  psychologists  present  us  with  no  end 
of  differences. 

The  criterion  of  consciousness  which  is  perhaps  most 
widely  adopted  at  the  present  time  is  the  ability  to  learn  by 
experience,  or  associative  memory.  Among  those  who  have 
adopted  this  standpoint  may  be  mentioned  Bethe,  Loeb 
and  Bohn.  Romanes  and  Lloyd  Morgan,  while  they 
recognize  that  ability  to  learn  is  indicative  of  the  presence  of 
consciousness,  hesitate  to  draw  the  conclusion  that  con- 
sciousness is  absent  in  those  animals  which  are  devoid  of 
associative  memory.  In  his  work  on  Animal  Intelligence 
Romanes  states  that  "because  a  lowly  organized  animal 
does  not  learn  by  its  own  individual  experience,  we  may 
not  therefore  conclude  that  in  performing  its  natural  or 
ancestral  adaptations  to  appropriate  stimuli  consciousness, 
or  the  mind  element,  is  wholly  absent;  we  can  only  say  that 
this  element,  if  present,  reveals  no  evidence  of  the  fact.^' 
Lloyd  Morgan  writes  likewise  in  a  guarded  manner :  "  When 
we  see  that  a  chick,  for  example,  pecks  at  first  at  any  small 
object,  it  is  difficult  to  say,  on  these  grounds,  whether  it  is 
a  sentient  animal  or  an  unconscious  automaton;  and  if  it 
continued  to  behave  in  a  similar  fashion  throughout  life, 
our  difficulty  would  still  remain.  But  when  we  see  that 
some  objects  are  rejected  while  others  are  selected,  we  infer 
that  consciousness  in  some  way  guides  its  behavior.  The 
chick  has  profited  by  experience.     But  even  this  is  only 


INTRODUCTION  5 

a  criterion  of  what  we  may  term  effective  consciousness. 
There  may  be  sentience  which  is  merely  an  accompaniment 
of  organic  action  without  any  guiding  influence  on  subse- 
quent modes  of  behavior.  In  that  case  it  is  not  effective; 
and  whether  it  is  present  or  not  we  have  no  means  of  ascer- 
taining." 

It  may  be  argued  that  since  learning  in  us  is  a  conscious 
process  that  therefore  all  animals  that  learn  are  likewise 
conscious.  The  inference  may  be  probable,  though  by  no 
means  necessary;  but  it  would  afford  no  ground  for  denying 
consciousness  in  animals  in  which  learning  does  not  occur. 
If  I  am  pricked  by  a  needle  I  am  acutely  conscious.  The 
feeling  of  pain  is  aroused  very  directly,  and  it  is  difficult  to  see 
how  it  can  be  dependent  in  any  way  on  associative  memory. 
If  my  memory  should  fail  me  to  the  extent  that  I  would 
straightway  forget  the  experience  every  time  I  was  pricked 
would  not  the  pricking  arouse  the  same  painful  sensation 
as  before?  Would  not  sound  waves  produce  the  sensation 
of  hearing  and  retinal  stimulation  the  sensation  of  light  in 
the  absence  of  any  power  of  recalling  similar  sensations 
received  in  the  past?  Unless  it  can  be  shown  that  there  is 
some  relation  of  dependence  of  consciousness  upon  associa- 
tive memory  in  ourselves  there  is  little  ground  for  setting 
up  associative  memory  as  a  criterion  of  consciousness  in 
animals.  The  criterion  reverses  the  obvious  relation  of 
the  phenomena.  Instead  of  consciousness  being  dependent 
upon  associative  memory,  associative  memory  implies  the 
previous  existence  of  consciousness.  How  can  the  exist- 
ence of  anything  be  dependent  on  the  association  of  its 
elements,  if  these  elements  can  exist  only  on  the  condition 
that  they  are  associated?  The  criterion  makes  the  exist- 
ence of  the  more  simple  and  general  dependent  on  the  exist- 
ence of  a  special  function  within  its  own  realm. 


6  INTRODUCTION 

One  reason  that  has  led  to  the  adoption  of  the  criterion 
of  associative  memory  is  the  evidence  afforded  that  the  animal 
at  this  stage  is  guided  by  experiences  of  pleasure  and  pain. 
We  repeat  acts  that  bring  us  pleasure  and  avoid  those  that 
bring  pain.  Where  in  animals  certain  responses  that  have 
been  made  once  tend  to  be  performed  more  readily  on  sub- 
sequent occasions  and  other  acts  which  are  followed  by 
avoiding  reactions  are  discontinued,  it  is  natural  to  infer 
that  pleasure  and  pain  accompany  these  different  modes 
of  behavior.  Pleasure  and  pain  have  very  commonly  been 
spoken  of  as  agents  of  reinforcement  and  inhibition.  W^hen 
the  pleasure-pain  response  appears  on  the  scene  conscious- 
ness is  commonly  assumed  to  take  a  guiding  hand  in  the 
determination  of  behavior.  The  advent  of  this  type  of 
behavior  marks  a  critical  period  in  the  evolution  of  the 
animal  mind  and  we  shall  consider  it  more  closely  in  a  sub- 
sequent chapter.  There  we  shall  attempt  to  show  that 
this  type  of  reaction  does  not  involve  the  injection  of  any 
radically  new  element  into  its  course  of  evolution.  If  we 
adhere  to  the  doctrine  of  psycho-physical  parallelism  in  any 
of  its  forms  we  cannot  speak,  in  strictness,  of  pleasure  and 
pain  as  agents  in  accommodation:  they  are  only  the  signs 
of  a  certain  kind  of  adjustment.  This  adjustment  according 
to  the  theory  of  parallelism  has  its  physiological  explanation 
without  calling  upon  the  interference  of  psychical  states. 
We  might  ask  broadly:  If  psychical  states  do  not  as  such 
interfere  with  the  course  of  physical  phenomena,  how  can 
we  adopt  any  kind  of  behavior  as  a  criterion  of  conscious- 
ness? I  doubt  if  either  Bethe  of  Loeb  is  willing  to  defend 
the  view  that  consciousness  is  an  agent  in  directing  physical 
phenomena,  and  to  accept  the  logical  consequences  of  such 
a  position.  It  can  be  shown,  I  believe,  that  this  view  creates 
many  serious  difficulties  without  giving  us  any  real  aid  in 


INTRODUCTION  7 

explaining  one.  But  a  discussion  of  this  question  in  all  its 
bearings  would  carry  us  too  far. 

If  there  is  no  infallible  test  for  the  determination  of  the 
existence  of  consciousness  in  animals  we  are  by  no  means  led, 
as  some  physiologists  would  have  us  believe,  to  the  denial 
of  the  possibility  of  a  comparative  psychology.  If  we  can 
explain  the  behavior  of  lower  organisms  in  terms  of  phy- 
siological laws  without  assuming  any  role  of  conscious  states 
in  determining  their  acts,  and  were  able  to  extend  the  same 
kind  of  explanation  to  higher  forms  until  ultimately  the 
whole  sentient  creation  were  embraced  in  our  system,  the 
rejection  of  comparative  psychology  would  logically  lead, 
as  Claparede  thas  well  shown,  to  the  denial  of  human  psy- 
chology as  well.  On  the  other  hand,  starting  in  with  the 
assumption  that  human  psychology  is  a  legitimate  subject 
matter  for  a  science,  there  is  no  logical  ground  for  refusing 
to  draw  inferences  concerning  the  mental  aspects  of  the 
behavior  of  apes,  and  other  higher  mammals;  and  if  we 
can  infer  something  of  the  mental  life  of  the  animals  near- 
est ourselves  we  are  warranted  in  extending  our  psycho- 
logical inferences  as  far  down  in  the  scale  as  analogy  per- 
mits us  to  go.  How  far  analogy  warrants  us  in  going 
is  capable  of  but  a  very  indefinite  answer.  When  we 
come  to  ants  and  spiders  everyone  may  perhaps  be  allowed 
to  have  his  own  opinion;  and  there  may  be  a  more  than 
sufficient  compensation  for  our  inabihty  to  reach  conclu- 
sive proofs  of  our  views  regarding  the  psychic  life  of  such 
creatures  in  having  a  perennial  source  of  contention  with 
which  comparative  psychologists  may  occupy  themselves 
whenever  they  come  together. 

The  existence  of  mind  in  the  lower  animals  has  something 
more  than  a  theoretical  interest.  In  our  experiments  on 
these  creatures  it  would  be  unfortunate  if  we  were  mistaken 


8  INTRODUCTION 

in  regarding  them  as  unconscious  automata  and  should 
proceed  to  cut  them  up  in  a  ruthless  fashion  on  that  as- 
sumption. It  may  be,  as  Norman  has  attempted  to  show, 
that  pain  sensations  in  the  lower  invertebrates  are  not 
acute  at  most,  and  may  be  absent  entirely,  but  not  much 
would  be  lost  by  giving  the  animals  the  benefit  of  the 
doubt  so  far  as  is  consistent  with  attaining  the  objects  of 
experimental  work. 

The  scientific  instinct  for  continuity  tempts  us  to  ascribe 
some  form  of  sentiency  to  all  living  forms.  Certainly  it  is 
not  possible  to  disprove  the  existence  of  consciousness  of 
some  sort  in  any  organism.  In  the  lowest  animals  it  may 
be  nothing  more  than  a  very  dull  form  of  sensibility.  If  we 
would  avoid  the  assumption  of  an  absolute  beginning  of 
consciousness  we  may  hold  that  something  akin  to  con- 
sciousness, but  very  much  more  primitive  than  any  of  the 
forms  of  it  with  which  we  are  acquainted,  exists  in  connec- 
tion with  all  inorganic  processes.  Such  a  position  affords 
a  consistent  viewpoint  and  has  been  accepted  by  many  as 
enabling  us  to  conceive  the  whole  process  of  psychic  evolu- 
tion as  one  without  any  breaks  or  discontinuities  anywhere 
in  the  series. 

Our  task  in  the  present  volume  is  to  sketch  briefly  the 
evolution  of  behavior  from  its  simplest  manifestations 
in  the  reflex  actions  of  the  lowest  organisms  to  its  more 
elaborate  expressions  in  the  higher  mammals.  No  attempt 
has  been  made  to  give  a  survey  of  behavior  in  all  the  groups 
of  the  animal  kingdom.  Beginning  with  simple  reflex  action 
we  have  described  those  forms  of  behavior  which  are  com- 
monly called  tropisms  and  which  are  by  many  regarded  as  a 
comparatively  direct  outcome  of  reflex  nritability.  After 
an  account  of  behavior  of  the  lowest  members  of  the  ani- 
mal kingdom  we  have  given  a  general  account  of  instinct, 


INTRODUCTION  9 

the  analysis  of  instinct  into  simpler  processes,  and  the 
different  views  of  the  origin  and  evolution  of  instinct. 
Considerable  space  is  devoted  to  the  transition  between  in- 
stinct and  intelligence,  and  the  evolution  along  the  lines 
of  plasticity  and  ready  modifiabihty  of  behavior  which  has 
prepared  the  way  for  the  appearance  of  inteUigence.  This 
is  followed  by  a  general  and  confessedly  incomplete  sketch 
of  the  gradual  evolution  of  inteUigence  in  the  animal  king- 
dom, and  a  discussion  of  some  of  the  evidences  for  the  ex- 
istence in  the  higher  animals  of  a  certain  power  of  making 
inferences  which  some  modem  psychologists  have  claimed 
is  not  found*  in  any  sentient  creature  below  man. 

A  comparative  survey  of  the  actions  of  animals  shows  us 
that  behavior  is  very  much  the  same  sort  of  thing  wherever 
found.  It  is  one  of  the  many  valuable  contributions  of  the 
great  pioneer  in  genetic  psychology,  Herbert  Spencer,  to 
have  shown  the  fundamental  unity  of  biological  and  psycho- 
logical processes  in  all  their  varied  manifestations.  Instinct, 
memory,  volition  and  reason  are  all  parts  of  that  general 
process  of  adjustment  of  the  organism  to  its  environment, 
in  which  life  in  all  its  stages  essentially  consists.  As  we 
pass  from  lower  to  higher  forms  we  have  an  increase  in  the 
complexity  and  perfection  of  the  adjustments;  the  corre- 
spondence increases  in  space  and  in  time,  in  deiiniteness  and 
in  generality,  but  everywhere  it  is  "the  adjustment  of 
internal  relations  to  external  relations."  This  conception 
of  mental  life  marks  an  important  advance  upon  the  psy- 
chology current  in  Spencer's  early  years;  it  set  aside  the 
arbitrary  distinctions  of  the  so-called  "faculties"  and  pre- 
pared the  way  for  a  clearer  insight  into  the  gradual  unfold- 
ing of  mind  which  the  labors  of  genetic  psychologists  are 
slowly  giving  us. 


10  INTRODUCTION 

BIBLIOGRAPHY 

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Claparede,    E.    Les    animaux    sont    ils-conscients?     Rev.  Philos., 

51,  24,  '01.     La  psychologie  compar^e  est-elle  legitime?     1.  c. 

Arch,  de  Psych.,  5,  13,  '05. 
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and  its  History.    '74. 
Loeb,  J.     Comparative  Physiology  of  the  Brain  and  Comparative 

Psychology.     N.  Y.,  '04. 
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London,  '94.     Animal  Behaviour.     London,  '00. 
Romanes,  G.  J.    Animal  Intelligence.    N.  Y.,  '83.     Mental  Evo- 
lution in  Animals.     London,  '85. 
Spencer,  H.    Principles  of  Psychology,  3d.  ed.    N.  Y.,  '94. 
TiTCHENER,  E.  B.   Were  the  Earliest  Organic  Movements  Conscious 

or  Unconscious?  Pop.  Sci.  Mon.,  60,  458,  '02. 
Washburn,  M.  F.  The  Animal  Mind.  N.  Y.,  '09. 
Wasmann,    E.    Instinkt   und   Intelligenz    im   Tierreich,    3d.    ed. 

Freiburg,  i.  B.,  '05. 


CHAPTER  II. 

REFLEX  ACTION. 

"Pure  reflexes  are  admirably  adapted  to  certain  ends.  They  are 
reactions  which  have  long  proved  advantageous  in  the  phylum  of 
which  the  existent  individual  is  a  representative  embodiment. 
Perfected  during  the  course  of  ages,  they  have  during  that  course 
attained  a  stability,  a  certainty,  and  an  ease  of  performance  beside 
which  the  stabtlity  and  facility  of  the  most  ingrained  habit  acquired 
during  an  individual  life  is  presumably  small." — Sherrington, 
The  Integrative  Action  of  the  Nervous  System. 

The  term  reflex  action  is  used  in  a  wide  and  somewhat 
indefinite  sense,  but  it  is  applied  in  general  to  responses  of  the 
organism  which  are  evoked  directly  upon  the  application  of 
a  stimulus.  In  a  typical  reflex  we  have  a  sensory  or  afferent 
impulse  set  up  in  some  sense  organ  and  proceeding  toward 
a  ganglion  or  larger  nerve  center.  From  the  latter  an  efferent 
impulse  passes  outward  along  a  motor  nerve,  causing  a 
muscular  contraction.  Frequently  the  impulse  is  reflected 
back  near  its  point  of  origin,  more  or  less  after  the  fashion 
in  which  light  may  be  reflected  back  to  its  source  from  a 
mirror  and  the  reaction  occurs  in  the  stimulated  part.  It  is 
from  analogy  with  the  reflection  of  light  that  the  term  re- 
flex action  is  chosen,  although  the  analogy,  it  must  be  con- 
fessed, is  rather  loose,  and  in  some  respects  misleading. 
When  one  involuntarily  winks  the  eyes  after  they  are 
suddenly  illuminated  by  a  glare  of  light,  impulses  are  set 
up  in  the  retina  which  proceed  along  the  optic  nerve  to  the 
brain.  Here  they  traverse  certain  established  connections 
among  the  fibers,  by  means  of  which  they  become  directed 
outward  along  the  motor  nerves  to  the  muscles  of  the  eye- 

11 


12  REFLEX  ACTION 

lids  causing  them  to  contract.  This  process  takes  place  in 
a  small  fraction  of  a  second  after  stimulation,  and  occurs 
not  only  without  the  action  of  the  will,  but  even  in  spite 
of  efforts  to  prevent  it. 

Another  reflex  act,  the  so-called  tendon  reflex,  may  readily 
be  observed  by  striking  the  large  tendon  which  is  inserted 
into  the  patella  just  below  the  knee.  If  the  leg  is  hanging 
loosely  a  light  blow  on  the  tendon  will  cause  the  foot  to  be 
suddenly  extended.  Here  the  impulses  set  up  by  the  blow 
proceed  to  the  spinal  cord,  and  are  returned  along  motor 
nerves  to  the  extensor  muscles  of  the  leg  causing  them  to 
contract.  If  the  nervous  centers  of  the  spinal  cord  are  im- 
paired, as  occurs  in  certain  diseased  conditions,  this  reflex 
can  no  longer  be  evoked. 

In  a  frog  these  spinal  reflexes  may  readily  take  place  if  the 
spinal  cord  is  cut  across  a  short  distance  in  front  of  the  center 
which  supplies  nerves  to  the  parts  concerned.  A  stimulus 
supplied  to  the  hind  leg  of  such  a  frog  causes  the  leg  to  be 
withdrawn.  If  in  a  frog  whose  brain  has  been  destroyed  the 
side  of  the  body  is  stimulated  by  acid  there  may  be  produced, 
besides  the  twitching  of  the  muscles  near  the  stimulated  part, 
a  movement  forward  of  the  hind  foot  of  the  same  side  which 
often  succeeds  in  wiping  away  the  acid.  If  the  stimulus  is 
applied  near  the  middle  of  the  body  both  hind  feet  may 
be  brought  forward  to  remove  it.  And  if  the  stimulus  be 
applied  to  one  side  and  the  leg  of  the  same  side  be  held,  the 
opposite  leg  may  be  brought  into  play,  especially  if  the 
stimulus  is  a  strong  one,  and  may  succeed  in  removing  the 
offending  substance.  The  accuracy  of  these  movements, 
especially  in  the  crossed  reflex,  is  exaggerated  in  most 
accounts,  but  sufficient  exactness  is  frequently  attained  to 
effect  the  removal  of  the  irritating  material. 

Reflex  action  may  occur  in  various  degrees  of  complexity. 


REFLEX  ACTION  13 

In  the  simplest  cases  there  is  movement  in  the  single  part 
affected,  as  when  a  single  tentacle  of  a  polyp  is  withdrawn 
upon  being  stimulated.  In  other  cases  a  single  stimulus 
may  produce  combined  movements  of  several  parts,  as  when 
the  various  appendages  of  a  brainless  crayfish  are  set  in  mo- 
tion when  but  one  of  them  is  stimulated.  A  stimulus  on  the 
right  side  of  a  frog  may  not  only  cause  coordinated  action  in 
the  various  muscles  of  the  hind  leg,  but  the  right  fore  leg 
may  be  apphed  to  the  stimulated  part  as  well.  And  if 
the  animal  is  rendered  especially  responsive  by  the  injection 
of  a  solution  of  strychnine,  contraction  may  take  place  in 
most  of  the  muscles  of  the  body. 

One  factor  which  is  concerned  in  the  complexity  of  the 
response  is  the  strength  of  the  stimulus.  A  Hght  stimulus 
apphed  to  the  tentacle  of  a  jelly  fish  usually  causes  a  contrac- 
tion of  that  tentacle  only,  but  if  the  stimulus  is  stronger, 
the  neighboring  tentacles  will  also  contract,  and  if  still 
stronger,  movements  may  be  set  up  in  the  umbrella  or  disk 
which  cause  the  animal  to  swim  away.  Complex  responses 
are  also  produced  if  a  stimulus  affects  many  sense  organs  at 
once. 

A  further  method  of  comphcation  occurs  in  the  so-called 
chain  reflexes  in  which  one  action  serves  as  the  stimulus 
for  a  second,  and  this  for  a  third,  and  so  on.  A  frog  with 
its  cerebral  hemispheres  removed  will  respond  to  the  move- 
ments of  a  fly  in  its  vicinity  by  snapping  at  it.  This  com- 
pHcated  reflex  causes  the  fly  to  be  seized  by  the  jaws;  the 
stimuli  caused  by  the  juices  of  the  fly  acting  on  the  taste 
buds  of  the  frog's  mouth  set  up  the  swallowing  reflex,  which 
involves  complex  and  coordinated  movements  of  the  tongue 
and  throat.  When  in  the  stomach  the  fly  reflexly  excites 
this  organ  to  activity,  resulting  finally  in  the  extrusion  of  the 
digested  mass  into  the  intestine,  which  in  turn  is  reflexly 


14  REFLEX  ACTION 

excited  to  the  performance  of  peristaltic  movements  and 
other  functions. 

Similarly  a  brainless  crayfish  will  seize  a  bit  of  food  in 
its  small  chela,  pass  it  forward  to  the  maxillipeds,  which 
cooperate  to  bring  it  between  the  mandibles  where  it  is 
chewed,  after  which  it  is  swallowed,  passing  into  the 
stomach,  where  it  sets  in  action  the  gastric  mill  resulting 
in  the  further  trituration  of  the  food. 

Reflex  actions  may  be  inhibited  in  various  ways.  If  the 
spinal  cord  of  a  frog  is  severed,  and  a  stimulus  applied  to  the 
anterior  cut  end  at  the  same  time  the  hind  toe  is  pinched, 
the  withdrawal  of  the  leg  which  occurs  regularly  under  nor- 
mal conditions  may  be  prevented  from  taking  place.  Im- 
pulses passing  down  the  cord  block  or  inhibit  impulses  tend- 
ing to  pass  down  the  motor  nerves  to  the  leg.  Strong  stimuli 
applied  to  the  optic  lobes  or  certain  other  parts  of  the  brain 
of  a  frog  produce  the  same  effect.  Reflex  acts  in  animals 
generally  take  place  more  readily  and  more  uniformly  if  the 
brain  be  destroyed,  indicating  that  the  brain  acts  as  a  con- 
stant inhibitory  organ  upon  the  lower  nerve  centers.  Stim- 
uli applied  to  other  parts  of  the  body  may  also  inhibit 
reflexes.  If  one  toe  of  a  decapitated  frog  is  strongly  stimu- 
lated by  being  held  in  dilute  sulphuric  acid  and  the  other 
toe  stimulated  by  an  electric  current,  the  withdrawal  of  the 
latter  will  be  delayed  or  entirely  checked. 

To  designate  as  reflex  action  the  direct  responses  of  the 
lowest  animals  which  are  devoid  of  a  nervous  system  is  to  ex- 
tend somewhat  the  original  meaning  of  the  term.  These  re- 
sponses, however,  are  now  commonly  spoken  of  as  ^'reflexes" 
and  they  are  very  similar  to  the  simple  reflex  acts  of  higher 
forms  in  which  a  nervous  arc  forms  the  pathway  of  the 
impulses.  To  a  certain  extent  all  living  matter  conducts 
stimuli  and  hence  performs  the  function  of  nervous  tissue. 


REFLEX  ACTION  15 

In  the  Protozoa  this  transmission  may  take  place  with  great 
readiness,  although  there  are  not  known  to  be  any  specialized 
pathways  which  the  impulses  follow.  If  the  bell-animalcule 
Vorticella  is  touched,  the  long  flexible  stalk  will  quickly 
contract.  A  Stentor  when  stimulated  contracts  with  great 
suddenness,  and  Paramoecium  when  it  encounters  a  stimulus 
of  any  kind  performs  a  stereotyped  motor  reflex  which  con- 
sists in  reversing  the  stroke  of  the  cilia,  swimming  backward, 
and  turning  toward  the  aboral  side.  This  response  is  shown 
even  by  small  pieces  of  the  infusorian  in  much  the  same  way. 
Most  of  the  protozoa  react  to  stimuli  by  some  form  of  motor 
reflex.  Even  plants  show  reflexes,  as  in  the  drooping  of  the 
leaves  of  the  sensitive  plant,  the  curling  of  the  tentacles  of  the 
sundew,  the  movements  of  the  Venus*  fly  trap,  and  many 
other  less  manifest  exhibitions  of  irritabihty. 

A  conspicuous  trait  of  reflexes  is  their  adaptiveness.  The 
winking  of  the  eye,  the  withdrawal  of  the  frog's  leg,  the  re- 
moval of  acid  from  the  side  of  the  body,  the  snapping  and 
swallowing  of  food  are  all  actions  which,  in  one  way  or  an- 
other, secure  the  welfare  of  the  individual.  But  the  pur- 
posiveness  is  like  that  of  a  machine  which  is  constructed  to 
do  a  particular  thing;  it  is  the  result  of  conformation  and 
arrangement  of  its  parts.  By  pressing  a  lever  a  machine 
may  be  made  to  perform  a  very  simple  act  or  produce  an 
elaborate  piece  of  music,  according  to  the  nature  of  the 
mechanism.  Given  the  proper  organization  an  animal  may 
likewise  perform  acts  of  great  complexity  and  adaptiveness 
in  response  to  certain  kinds  of  stimulation;  the  purposive- 
ness  of  the  behavior  results  not  from  choice,  but  from  a  sort 
of  mechanical  necessity.  The  explanation  of  this  adap- 
tiveness, like  the  explanation  of  the  adaptiveness  of  a 
machine  to  perform  a  certain  kind  .of  work,  lies  in  the 
causes   that  led   to  the   production  of   a  certain  kind   of 


16  REFLEX  ACTION 

structure.    For  a  solution  of  this  problem  we  are  naturally 
led  to  the  study  of  the  evolution  of  organic  life. 

RHYTHMICAL  ACTIVITY 

Organisms  perform  many  actions  repeatedly  which  appar- 
ently go  on  by  themselves  independently  of  outer  stimulation. 
Such  actions  may  be  viewed  as  responses,  but  the  stimuli 
causing  them  are  internal  instead  of  external.  The  beating 
of  the  heart  and  the  regular  movements  of  respiration  are 
familiar  examples  of  such  rythmical  activity. 

In  the  horse-shoe  crab  Limulus  the  abdominal  appendages 
Which  bear  the  gills  execute  a  regular  to  and  fro  movement 
which  will  occur  as  well  if  the  nerve  cord  is  cut  between  the 
thorax  and  the  abdomen.  Even  if  the  nerve  cord  is  cut 
between  the  several  abdominal  ganglia  the  appendages  will 
still  beat  regularly,  although  all  coordination  of  their 
movements  is  destroyed.  Among  medusae  there  is  a  regular 
contraction  of  the  disk  or  bell  during  swimming  which 
usually  disappears  if  the  marginal  nerve  ring  is  removed, 
but  Loeb  has  shown  that  if  the  jelly  fish  Gonionemus,  after 
being  operated  on  in  this  manner,  is  placed  in  sea  water 
devoid  of  calcium  salts  the  rhythmical  pulsations  will 
reappear. 

Among  the  Protozoa  rhythmical  movements  are  shown  in 
the  regular  swa5dng  of  Stentor,  and  the  periodical  contrac- 
tions of  Vorticella;  and  Loxophyllum,  as  is  described  in  a 
later  chapter,  shows  a  rhythmical  alternation  of  movements 
which  are  performed  in  much  the  same  way  by  very  small 
pieces  into  which  the  inf usorian  may  be  divided. 

BIBLIOGRAPHY 

Bbthe,  a.  Das  Centralnervensystem  von  Carcinus  moenas.  Arch, 
f.  mik.  Anat.,  50  and  51,  '97.  AUgemeine  Anatomic  und  Physi- 
ologic des  Nervensystems.     Leipzig,  '03. 


RHYMTHMICAL  ACTIVITY  17 

LoEB,  J.     Comparative  Physiology  of  the  Brain  and  Comparative 

Psychology.     N.  Y.,  '00. 
LuKAS,  F.  Psychologic  der  niedersten  Tiere.    Wien  and  Leipzig, 

'05. 
ScHRADER,  M.  Zur  Physiologio  des  Froschgehirns.    Pfltiger's  Archiv, 

41,  75,  '87. 
Sherrington,  C.  S.    The  Integrative  Action  of  the  Nervous  System. 

N.  Y.,  '03. 
Uexkxjll,  J.  von.    Umwelt  und  Innenwelt  der  Tiere.    Berlin,  '09. 


CHAPTER  III 
THE  TROPISMS 

"The  understanding  of  complicated  phenomena  depends  upon  an 
analysis  by  which  they  are  resolved  into  their  simple  elementary 
compounds." — Loeb,  Physiology  of  the  Brain. 

"On  s'est  fait  beaucoup  d'idees  fausses  sur  les  tropismes;  les 
litterateurs,  les  philosophes;  les  savants  meme,  ont  dissert^  sur  eux 
d'une  facon  tout  a  fait  fantaisiste,  les  uns  raillant,  les  autres  louant 
sans  reserve,  ce  qu'ils  n'avaient  pas  compris." — Georges  Bohn. 

THE  TROPISMS  IN  GENERAL 

Certain  stimuli  exercise  a  directive  effect  upon  the  move- 
ments of  animals  causing  them  to  go  toward  or  away  from 
the  source  of  stimulation.  Such  movements  are  commonly 
called  tropisms.  The  flight  of  a  moth  toward  the  candle, 
the  gathering  of  male  moths  around  a  box  containing  a 
female,  the  movements  of  protozoans  away  from  regions  of 
unusual  heat  or  cold  may  be  taken  as  illustrations  of  such 
directed  movements. 

No  attempt  will  be  made  to  formulate  an  accurate  defini- 
tion of  the  word  tropism  because  usage  has  not  established 
the  meaning  of  the  term  with  sufficient  precision  to  render 
this  possible.  There  is  considerable  difference  of  opinion 
in  regard  to  the  explanation  of  tropisms,  and  the  kinds  of 
behavior  to  which  the  term  should  be  applied.  In  what 
follows  phenomena  will  be  described  which  have  been  com- 
monly classed  as  tropisms  however  diverse  they  may  be  in 
character  and  causation. 

The  study  of  tropisms,  which  has  attracted  a  large  share 
of  attention  in  recent  years,  was  given  a  great  stimulus  by 

18 


THE  TROPISMS  IN  GENERAL  19 

Loeb  through  his  work  "Der  HeUotropismus  der  Tiere'^  and 
various  later  papers.  The  ''HeHotropismus  der  Tiere" 
was  a  pioneer  work;  but  perhaps  its  chief  value  Ues  not  so 
much  in  the  number  of  interesting  facts  and  records  of 
ingenious  experiments  it  contains  as  in  the  general  theory- 
there  advanced,  which  has  been  the  means  of  stimulating 
many  investigators  to  enter  the  field. 

The  book  emphasized  the  idea  that  it  is  the  business  of 
the  investigator  not  merely  to  describe  the  facts  of  behavior 
and  marvel  over  their  wonderful  nature,  but  to  try  to  ex- 
plain them.  It  presented  him  with  a  theory  which  if  true 
would  carry  us  a  step  forward  in  the  analysis  of  many 
kinds  of  behavior,  especially  in  the  lower  organisms.  The 
book  shows  the  influence  of  Sachs,  whose  conception  of  the 
mechanism  of  tropisms  in  plants  forms  the  nucleus  of  Loeb^s 
theory.  Loeb  attempted  to  show  that  the  phenomena  of 
Heliotropism  were  essentially  similar  in  plants  and  animals. 
The  orientation  of  the  body  to  the  direction  of  the  rays,  the 
fact  that  the  more  refrangible  rays  are  the  more  effective  in 
producing  the  heliotropic  response,  and  the  dependence  of 
light  reactions  upon  light  intensity  and  temperature  are 
shown  by  plants  and  animals  in  much  the  same  way.  All 
of  these  effects  are  supposed  to  rest  upon  certain  funda- 
mental properties  of  living  substance  common  to  plants  and 
animals.  This  conclusion  was  naturally  a  somewhat  start- 
ling one,  especially  when  the  light  reactions  of  highly  de- 
veloped animals  which  were  commonly  supposed  to  be  due  to 
psychic  causes  were  placed  in  the  same  category  with  the 
turning  of  leaves  and  stems  toward  the  sunlight. 

The  prospect  of  finding  a  mechanical  explanation  of  the 
behavior  of  organisms  is  always  an  alluring  one.  In 
this  case  it  proved  to  be  especially  so  because  the  general- 
ization included  such  apparently  diverse  phenomena,  and 


20  THE  TROPISMS 

could  be  applied  with  only  a  slight  modification  to  other 
kinds  of  tropisms.  "These  tropisms,"  says  Loeb,  ''are 
identical  for  animals  and  plants.  The  explanation  of  them 
depends  first  upon  the  specific  irritability  of  certain  elements 
of  the  body-surface,  and  second,  upon  the  relation  of  sym- 
metry of  the  body.  Symmetrical  elements  at  the  surface  of 
the  body  have  the  same  irritability;  unsymmetrical  elements 
have  a  different  irritability.  Those  nearer  the  oral  pole 
possess  an  irritabiUty  greater  than  that  of  those  near  the 
aboral  pole.  These  circumstances  force  an  animal  to  orient 
itself  in  such  a  way  that  symmetrical  points  on  the  surface 
of  the  body  are  stimulated  equally.  In  this  way  the  animals 
are  led  without  will  of  their  own  either  toward  the  source  of 
stimulus  or  away  from  it.  Thus  there  remains  nothing  for 
the  ganglion  cell  to  do  but  to  conduct  the  stimulus,  and  this 
may  be  accomplished  by  protoplasm  in  any  form." 

If,  for  instance,  a  worm  were  near  a  bit  of  food  we  might 
suppose  that  the  substances  diffusing  from  the  food  would 
strike  one  side  of  the  body  of  the  worm  causing  the  muscles 
there  to  contract  more  strongly  than  the  opposite  ones. 
The  worm  would  turn  toward  the  food  until  both  sides 
of  the  body  were  equally  affected,  when  it  would  proceed 
directly  toward  the  source  of  stimulation.  Negative  re- 
sponses receive  an  essentially  similar  interpretation.  The 
point  that  is  emphasized  by  the  theory  is  that  choice  or 
volition  on  the  part  of  the  animal  is  excluded;  the  actions  of 
the  creature  are  supposed  to  be  mechanically  determined. 
It  is  "forced"  to  go  toward  or  away  from  the  source 
of  stimulation  as  the  strength  of  the  stimulus  and  the 
organization  of  its  body  determine.  The  moth  flies  to  the 
candle,  not  because  it  is  drawn  by  curiosity,  as  suggested 
by  Romanes,  or  from  any  other  conscious  motive,  but 
because  it  is  compelled  to  orient  its  body  so  that  sym- 


THE  TROPISMS  IN  GENERAL  21 

metrical  points  receive  equal  amounts  of  stimulation. 
The  orientation  of  animals  to  heat,  chemical  substances, 
gravity,  the  electric  current  and  currents  of  air  and  water 
is  naturally  explained  in  much  the  same  way.  Animal 
instincts  may  be  analyzed  into  simple  tropisms  or  may 
be  conceived  to  be  developed  from  them  as  a  basis. 

The  tropism  theory  of  Loeb  and  his  followers  has  met  with 
a  certain  amount  of  opposition,  especially  by  Jennings, 
who  showed  that  in  many  of  the  so-called  tropisms  of  the 
lower  organisms  there  is  no  definite  orientation  produced. 
In  the  gathering  of  Paramoecia  in  weak  acid,  for  instance, 
the  organisms  are  not  forced  into  line  with  the  diffusion 
currents  and  compelled  to  swim  toward  the  chemical,  but 
individuals  which  swim  into  the  acid  by  chance  remain 
there;  whenever  they  attempt  to  pass  from  the  dilute  acid 
to  water  they  reverse  their  course,  and  thus  are  kept  con- 
fined to  one  region.  While  the  chemotactic  grouping  of 
P^tramoecia  depends  upon  a  definite  reflex,  it  is  produced  in  a 
manner  quite  different  from  Loeb's  scheme  of  orientation. 
Many  of  the  so-called  tropisms  of  the  infusoria  and  other 
asymmetrical  forms  were  found  to  take  place  in  accordance 
with  the  method  followed  by  Paramoecium. 

In  some  cases  where  orientation  is  effected  it  may  take 
place  more  or  less  indirectly  by  the  selection  of  random 
movements.  In  the  earthworm  and  in  the  larvae  of  blow 
flies  which  are  negatively  phototactic  it  has  been  shown  by 
the  writer  that  movements  which  bring  the  animal  toward 
the  light  are  checked  or  reversed  and  only  those  which  hap- 
pen to  direct  the  animal  away  from  the  light  are  followed  up. 
Whatever  immediate  orienting  tendency  the  light  may  have 
in  these  cases  is  relatively  unimportant  as  compared  with 
the  element  of  selection  of  favorable  chance  movements  in 
directing  the  animal  away  from  the  light.    The  tropism 


22  THE  TROPISMS 

does  not  as  in  Paramoecium  occur  independently  of  orienta- 
tion, but  the  orientation  is  only  indirectly  produced  instead 
of  being  the  result  of  appropriate  direct  reflexes. 

The  movements  of  organisms  toward  or  away  from  certain 
stimulations,  even  where  they  are  quite  directly  caused, 
may  be  brought  about  in  a  variety  of  ways.  They  are  by 
no  means  always  the  result  of  a  forced  orientation.  Never- 
theless such  movements  are  commonly  described  as  tropisms. 
Whether  or  not  the  tropic  response  is  effected  in  one  or  the 
other  of  the  ways  we  have  described  it  may  be  just  as  much 
the  result  of  reflex  action  and  just  as  little  the  outcome  of 
intelligence  and  volition. 

CHEMOTAXIS 

Movements  toward  or  away  from  chemicals  are  naturally 
widespread.  Many  kinds  of  bacteria  are  markedly  che- 
motactic.  Englemann  found  that  Bacterium  termo  gathers 
near  the  margin  of  the  cover-glass  where  there  is  more 
oxygen  than  near  the  center.  If  green  algae  are  present  the 
bacteria  congregate  around  them  as  long  as  they  are  exposed 
to  sunlight  and  are  therefore  giving  off  oxygen.  If  the  slide 
is  kept  in  the  dark  so  that  no  more  oxygen  is  produced  and 
the  oxygen  present  becomes  evenly  diffused  the  bacteria 
become  uniformly  scattered  over  the  field.  Englemann 
in  an  ingenious  experiment  also  showed  that  if  a  spectrum 
were  thrown  on  a  long  thread  of  the  alga  Cladophora  the 
bacteria  would  congregate  most  abundantly  in  the  red  end 
of  the  spectrum  where  most  oxygen  is  given  off.  The  ex- 
periment affords  a  delicate  test  of  the  amount  of  oxygen 
evolved  under  the  influence  of  the  various  kinds  of  light. 

Bacterium  termo  and  other  species  were  found  by  Pfeffer 
to  be  attracted  to  various  salts  if  they  were  employed  in 
weak  solution,  as  well  as  a  variety  of  other  substances  of  the 


CHEMOTAXIS 


23 


most  diverse  chemical  constitution.  Many  compounds  were 
found  to  exercise  a  repellent  effect,  especially  in  strong  con- 
centration. Amoeba  reacts  negatively  to  most  chemicals 
that  affect  it.  ^\Tiere  it  comes  in  contact  with  the  diffusing 
chemical  the  ectoplasm  contracts;  currents  beginning  in  the 
stimulated  region  gradually  extend  through  the  body; 
pseudopods  are  thrust  out  opposite  the  stimulated  part 
and  the  Amoeba  begins  to  crawl  away.  Positive  chemo- 
taxis  probably  plays  a  part  in  the  food  taking  of  Amoeba, 
although  its  r61e  has  not  been  clearly  worked  out.    In  the 


FiQ.  1. — A.   B.  C.  Sucessive    phases  of   the  reaction  of  Amoeba  to  a 
chemical  (represented  by  the  dotted  area). 

amoeboid  plasmodia  of  myxomycetes,  which  usually  live  upon 
decaying  wood,  Stahl  has  found  that  extract  of  bark  pro- 
duces a  positive  response. 

Chemotactic  reactions  are  as  a  rule  pronounced  among 
the  infusoria.  Paramcecia  and  some  other  forms  collect  in 
weak  acids,  while  they  are  generally  negative  to  stronger  acids 
and  to  alkahes.  If  a  drop  of  strong  carbon  dioxide  solution 
is  placed  in  the  midst  of  a  lot  of  Paramcecia  the  infusorians 
will  form  a  ring  about  it.  Being  negative  to  the  stronger 
acid  at  the  center  of  the  drop  and  positive  to  the  weaker 
concentration  that  surrounds  it  they  naturally  collect  in  a 


24  THE  TROPISMS 

band  or  ring  whose  diameter  varies  with  the  size  and  strength 
of  the  drop.  As  the  solution  diffuses  and  becomes  diluted 
at  the  center,  the  ring  widens  and  extends  toward  the 
center  and  after  a  time  closes  in  to  form  a  soHd  group.  If 
two  species  of  protozoa  having  a  different  attunement  to 
carbon  dioxide  are  used  they  may  form  two  rings,  one  out- 
side of  the  other. 

The  tendency  of  Paramoecium  to  form  groups  may  be 
explained  as  due,  in  part  at  least,  to  their  positive  chemo- 
taxis  to  carbon  dioxide.  If  water  containing  Paramcecia 
is  allowed  to  stand  undisturbed  for  some  time  most  of  the 

individuals  will  be  found  in 
groups  or  clusters  as  if  they 
were  drawn  together  by  their 
social  proclivities  or  by  some 
object  of  common  interest.  But 
Fig.  2 —Showing  the  positive    the  cause  of  the  association  is 

chemotaxis  of  Paramoecium  to  i       •       i  * 

a  drop  of  a  weak  solution  of  car-    much  Simpler.    As  Paramcicea 

bon  dioxide.  .  «.  ,  j*      •  j      xv 

give  off  carbon  dioxide  there 
is  a  greater  quantity  of  it  in  a  region  where  several 
Paramcecia  happen  to  be  associated.  Positive  chemotaxis 
to  this  substance  tends  to  keep  the  cluster  together  as  well 
as  to  retain  other  individuals  which  happen  to  come  into 
the  region,  until  finally  most  of  the  Paramcecia  in  the  vicinity 
are  assembled  in  the  group.  Other  infusorians,  such  as 
Oxytricha  and  Loxocephalus,  form  associations  which  are 
not  due  to  carbon  dioxide  or  to  any  other  acids  since  they 
show  no  positive  chemotaxis  to  these  substances.  There 
is  probably  some  substance  which  produces  the  grouping, 
although  it  has  not  yet  been  identified. 

The  chemotaxis  of  the  lower  organisms  is  to  a  certain 
extent  related  to  the  welfare  of  the  individuals,  although  in 
only  a  general  sort  of  way.    The  negative  reaction  to  strong 


CHEMOTAXIS  25 

chemicals  is  a  purposive  trait,  although  many  forms  do  not 
react  away  from  solutions  which  are  decidedly  injurious. 
According  to  Massart  Polytoma  uveUa  may  not  be  repelled 
even  by  the  strongest  chemicals.  Other  organisms  while 
perfectly  able  to  swim  away,  do  not  seem  to  be  prepared  for 
meeting  such  a  situation  as  the  presence  of  injurious  chem- 
icals by  any  appropriate  response.  If  specimens  of  Loxo- 
phyllum  meleagris  and  Paramoecium  are  placed  together 
under  a  cover-glass  and  a  small  drop  of  acid  applied  to  one 
edge  the  Paramoecia  will  avoid  the  acid,  but  the  Loxophylla 
move  about  helplessly,  many  of  them  swimming  directly 
into  the  strong  acid,  others  which  may  be  overtaken  by  the 
acid  showing  no  more  tendency  to  swim  away  from  it  than 
toward  it.  The  result  is  that  nearly  all  of  the  Paramoecia 
in  the  region  of  the  diffusing  acid  escape,  while  nearly  all 
of  the  Loxophylla  perish.     y^.'vAr  v 

The  general  tendency  of  Paramoecium  to  collect  in  all  kinds 
of  weak  acid  and  to  react  negatively  to  alkalies  is  prob- 
ably of  little  service  to  the  organism.  Bacteria  show  a  posi- 
tive reaction  to  various  substances  which  they  never  encoun- 
ter under  normal  conditions  and  whose  presence  can  scarcely 
be  of  any  benefit  to  them.  On  the  other  hand,  the  positive 
reactions  of  several  species  to  oxygen,  as  well  as  the  negative 
reactions  of  certain  anaerobic  species,  are  doubtless  beneficial 
modes  of  response.  It  is  not  improbable  that  in  many 
cases  the  response  to  substances  that  are  not  beneficial  is 
the  incidental  result  of  the  organization  which  causes  posi- 
tive responses  to  substances  that  are  beneficial.  For 
instance,  it  may  be  advantageous  to  Paramoecium  to  be  con- 
stituted so  as  to  react  positively  to  weak  solutions  of  carbon 
dioxide,  and  its  positive  response  to  other  weak  acids  may 
depend  upon  the  same  peculiarity.  While  much  of  the 
chemotactic  behavior  of  lower  organisms  may  be  indifferent 


26  THE  TROPISMS 

as  regards  utility  or  may  be  positively  injurious,  a  consider- 
able amount  of  adaptiveness  is  shown,  especially  to  sub- 
stances which  are  habitually  encountered.  The  situation 
is  much  as  would  be  expected  if  starting  with  reactions  which 
primarily  showed  no  relation  to  utility  organisms  came  to 
have  their  irritability  modified  and  directed  along  service- 
able lines  to  the  extent,  and  only  to  the  extent  that  was 
necessary  to  insure  survival  under  the  conditions  to  which 
they  were  naturally  exposed. 

Chemotaxis  is  shown  by  parts  of  organisms,  especially 
the  free  cells  such  as  spermatozoa,  antherozoids  and  leuco- 
c\i;es.  Pfeffer  showed  that  the  spermatozoids  of  ferns 
would  collect  in  capillary  tubes  containing  a  dilute  solution 
of  mahc  acid  and  he  concluded  that  the  presence  of  this 
substance  in  the  archegonia  is  the  means  of  drawing  the 
spermatozoids  to  the  egg  cell  and  thus  effecting  fertilization. 
A  similar  role  is  supposed  to  be  played  by  cane  sugar  in 
mosses.  The  white  blood  cells  which  have  the  property  of 
crawling  about  much  like  Amoeba  are  attracted  by  certain 
substances,  and  often  gather  in  large  numbers  where  there 
is  a  bacterial  infection.  If  a  small  tube  containing  a  cultiu"e 
of  Staphylococcus  aureus  in  agar  is  placed  in  one  of  the  lymph 
spaces  of  a  frog  the  white  corpuscles  will  migrate  into  it  in 
large  numbers.  A  tube  with  the  same  culture  medium, 
but  without  the  bacteria  will  not  be  invaded,  showing  that 
it  is  the  presence  of  some  substance  given  off  by  the  bacteria 
which  causes  the  leucoc>i;es  to  enter  the  tube. 

The  chemotactic  movements  of  Amcsba  and  related 
forms  differ  markedly  from  the  scheme  followed  by  the  in- 
fusoria and  many  flagellates.  In  Amoeba  the  part  directly 
affected  is  the  first  to  act  and  there  is  a  more  or  less  definitely 
directed  series  of  movements  away  from  the  chemical. 
In  the  positive  reactions  of  Paramcecium  there  is  no  orienta- 


CHEMOTAXIS  27 

tion  of  the  body,  no  seeking  of  the  stimulus  and  no  attra<.-tion 
in  any  sense.  The  Paramoecia  as  they  happen  to  approach 
a  drop  of  dilute  chemical  to  which  they  are  positiveiy 
chemotactic,  s\\im  into  it  without  interruption,  but  when 
they  encounter  the  other  side  of  the  drop  and  experience 
the  transition  from  weak  acid  to  water  they  give  the  motor 
reflex  or  avoiding  reaction  and  change  their  course.  As 
often  as  the  Paramoecium  encounters  this  boundary  it 
reacts  in  the  same  way.  It  behaves  as  if  caught  in  a  trap 
which  presents  no  obstacle  to  its  entrance,  but  effectually 
prevents  its  exit.  Other  Paramoecia  sv^imming  by  chance 
into  the  drop  are  also  caught  there  and  soon  a  collection  is 
formed  which  may  ultimately  include  all  the  individuals  of 
the  region.  In  the  negative  response  the  method  is  much 
the  same  only  it  is  the  transition  from  weak  to  strong  acid 
which  causes  the  stimulation.  The  avoiding  reaction  is 
given  in  the  same  way  in  each  case.  All  of  the  chemotactic 
responses  take  place  without  any  orientation  of  the  body, 
by  a  method  of  trial  and  error.  ; 

In  the  flagellate  Chilomonas  Jennings  has  shown  that 
chemotactic  grouping  takes  place  just  as  in  ParamcBcium. 
In  the  bacterium  Spirillum  he  finds  much  the  same  method 
employed.  Spirillum  S'wims  by  the  movement  of  flagella 
at  one  or  both  ends  of  the  body.  \Mien  stimulated  it 
simply  reverses  its  direction  of  movement.  Positive  and 
negative  reactions  take  place  in  exactly  the  same  way. 
What  things  are  avoided  and  what  are  sought  depends 
upon  what  acts  as  a  stimulus  causing  the  reversing  reaction. 
Apparently  these  simple  organisms  have  no  power  of 
orienting  their  bodies;  they  simply  move  back  and  forth, 
relying  on  chance  to  get  them  into  a  favorable  situation. 
When  such  a  situation  has  been  reached  the  organisms 
remain  relatively  quiet.     In  several  cases  among  the  flagel- 


/ 


28  THE  TROPISMS 

lates  and  in  spermatozoids  and  swarm  spores  it  has  been 
supposed  that  there  is  a  direct  turning  toward  the  chemical. 
\  This  may  occur  in  some  forms  and  more  especiallly  the 
\  symmetrical  ones,  but  it  has  not  yet  been  established. 
Analogy  with  the  effect  of  light  on  Euglena  which  may  call 
forth  either  the  motor  reflex  or  a  gradual  swerving  of  its 
course  toward  the  stimulated  side  makes  the  existence  of  a 
parallel  response  to  chemical  stimulation  more  or  less 
probable. 

The  coelenterates  show  scarcely  any  behavior  which  can  be 
described  as  chemotactic  although,  reaction  to  chemicals  is  of 
course  general.  If  a  Hydra  is  locally  stimulated  by  a  strong 
chemical  which  is  allowed  to  diffuse  against  the  body  from 
a  fine  capillary  tube  there  is  contraction  of  the  muscles  in  the 
stimulated  area  causing  the  body  to  bend  toward  the  chem- 
ical. This  is  certainly  not  a  teleological  response;  rather 
the  reverse  of  one.  A  still  stronger  stimulus  however  will 
produce  a  general  contraction  of  the  body  which  enables  the 
animal  to  avoid  more  serious  injury.  In  planarians  both 
positive  and  negative  reactions  may  be  induced.  The  com- 
mon fresh- water  Planaria  maculata  may  be  made  to  follow  a 
piece  of  meat  around  in  any  desired  direction.  The  substances 
diffusing  from  the  meat  seem  to  stimulate  the  worm  to  turn 
its  head  directly  toward  the  food.  Strong  chemicals,  on 
the  other  hand,  cause  it  to  turn  directly  away.  Earth- 
worms, according  to  Darwin,  find  food  that  is  buried  under 
the  earth  through  the  sense  of  smell.  Disagreeable  sub- 
stances cause  contraction  and  writhing  about,  which  may 
bring  the  worm  out  of  the  region  of  the  offending  stimulus. 
A  strong  chemical  applied  to  one  side  of  the  body  may  cause 
the  animal  to  turn  toward  the  other  side,  but  the  avoidance 
of  such  stimuli  is  mainly  effected  by  general  and  random 
movements. 


GEOTAXIS  29 

Crustaceans  are  able  to  detect  food  at  some  distance  and 
direct  themselves  toward  it,  but  one  may  question  if  such 
phenomena  should  be  classed  under  the  head  of  true  chemo- 
taxis.  Similarly  with  the  olfactory  reactions  of  ants. 
These  insects  follow  a  scent  track  with  considerable  accuracy. 
In  this  way  they  may  be  guided  to  food  discovered  by  other 
ants  and  find  their  way  back  to  the  nest.  The  tendency  to 
follow  these  tracks  is  doubtless  instinctive,  and  so  also  is  the 
action  of  a  dog  in  following  the  trail  of  a  rabbit.  The  be- 
havior of  the  dog  is  on  a  much  higher  level  than  a  mere 
tropism,  and  it  is  probable  that  the  behavior  of  the  ant  is 
also,  but  to  a  less  degree.  But  where  to  draw  the  line 
between  such  actions  and  chemotaxis  proper  is  perhaps 
capable  of  only  an  arbitrary  decision. 

GEOTAXIS 

Gravity  exercises  a  directive  effect  upon  the  position  and 
movements  of  many  animals  while  in  other  forms  it  has  little 
orienting  power.  The  protozoans  Euglena  and  Chlamy- 
domonas  commonly  swim  upward  in  the  dark  as  well  as  in 
the  light,  but  this  reaction  is  checked  at  a  low  temperature 
of  5°  or  6°  C.  Massart  studied  the  geotactic  movements  of  a 
number  of  unicellular  plants  and  animals  by  placing  them  in 
a  capillary  tube  open  at  either  end  so  as  to  render  the  supply 
of  oxygen  the  same  above  and  below.  He  found  that  the 
sense  of  the  geotactic  response  varies  in  allied  species  of 
the  same  genus,  as  for  instance.  Spirillum  some  of  which 
are  positive  and  some  negative  under  the  same  conditions. 
The  sense  of  the  response  could  be  changed  in  some  cases  by 
temperature.  Thus  Chromulina,  which  is  negative  at  15® 
to  20°  C.,  becomes  positive  at  a  temperature  of  5°  to  7°  C. 

The  geotaxis  of  Paramoecium  is  quite  variable  and  often 
quite  feeble.     Generally  there  is  a  tendency  to  swim  up- 


30  THE  TROPISMS 

ward,  but  Miss  Moore  has  shown  that  lack  of  food,  changes 
of  temperature  and  other  factors  may  cause  a  reversal  of 
the  response.  The  infusorian  Spirostomum  has  the  peculiar 
trait  of  attaching  itself  to  the  bottom  by  means  of  a  mucous 
thread  at  the  posterior  end  and  orienting  itself  in  a  vertical 
position. 

Among  the  Ccelenterates  as  in  the  Protozoa  there  are 
many  forms  in  which  geotaxis  seems  entirely  absent.  The 
fresh  water  Hydra  will  attach  itself  to  the  bottom  or  side 
of  an  aquarium  or  hang  downward  from  the  surface  film 
with  apparently  equal  readiness.  Many  anemones  are 
equally  indifferent  to  their  position,  but  Sagartia,  according 
to  Torrey,  will  bend  upward  if  attached  to  the  side  of  an 
aquarium  and  slowly  migrate  to  the  top.  Loeb  found  that 
if  Cerianthus,  which  lives  with  the  lower  part  of  its  body 
buried  in  the  sand,  is  placed  in  an  inverted  position  in  a  test- 
tube  the  foot  will  curve  downward  and  the  bending  will 
gradually  continue  until  the  animal  finally  straightens  out 
into  an  upright  position. 

In  jellyfish  which  generally  swim  with  the  oral  side  down- 
ward orientation  has  been  attributed  to  the  statocysts  which 
occur  on  the  margin  of  the  umbrella.  In  Gonionemus, 
however,  Murbach  has  shown  that  orientation  is  unimpaired 
after  the  destruction  of  all  these  organs,  so  they  cannot  be 
the  exclusive  seat  of  the  reaction  to  gravity.  The  orienta- 
tion is  not  one  which  is  passively  assumed  for,  as  Murbach 
has  shown,  specimens  which  have  been  killed  float  with  the 
oral  surface  upward. 

In  the  Ctenophores  the  statocyst  which  is  located  at  the 
aboral  pole  of  the  body  is  a  more  essential  organ  of  equilib- 
rium, for  as  Verworn  has  shown  the  removal  of  this  organ 
is  followed  by  loss  of  orienation  to  gravity.  In  some 
Ccelenterates — Antennularia  (Loeb),  Sertulariella  (Driesch), 


GEOTAXIS  31 

Cormyorpha  (Torrey) — geotropism  is  manifested  in  the 
direction  of  growth  much  as  in  the  higher  plants. 

Among  the  worms  geotactic  migrations  are  manifested  by 
Convoluta  and  were  found  by  Bohn  to  vary  with  the  tide 
as  is  described  in  another  connection  (Chapter  VII).  Des- 
truction of  the  statoHths  in  this  form  was  found  to  destroy 
its  reaction  to  gravity.  The  sea-cucumber  Cucumaria 
cucumis  is  described  by  Loeb  as  having  a  marked  tendency 
to  crawl  upward.  The  upward  migration  occurs  as  well  in 
inverted  glass  cylinders,  showing  that  it  is  not  the  supply 
of  oxygen  which  attracts  the  animal  to  the  upper  surface. 

A  tendency  to  vertical  migration  is  shown  by  many  free 
swimming  Crustacea.  In  the  copepod  Labidocera  Parker 
observed  that  the  females  showed  a  marked  negative  geo- 
taxis  in  the  dark,  but  when  exposed  to  light  their  geotaxis 
became  positive.  Whether  they  were  illuminated  from  above 
or  below  did  not  alter  their  response.  Although  negatively 
phototactic  they  nevertheless  swim  downward  when  light 
falls  on  them  from  below. 

Many  crustaceans  have  statocjsts  which  are  especially 
concerned  in  the  maintenance  of  equilibrium.  In  Mysis 
they  are  located  in  the  inner  branch  of  the  caudal  appendages. 
Delage  found  that  if  these  organs  were  destroyed  the  animals 
had  difficulty  in  maintaining  their  equilibrium,  which  was 
further  increased  by  the  destruction  of  the  eyes.  In  the 
decapod  Crustacea  the  statocysts  are  situated  in  the  basal 
joint  of  the  antennules.  Destruction  of  these  organs  is 
followed  by  a  variable  degree  of  loss  of  equilibrium  in  differ- 
ent species. 

The  most  instructive  and  ingenious  experiments  in  the 
functions  of  the  statocysts  were  performed  by  Ki-eidl  on  the 
prawn  Palaemon.  In  moulting,  the  lining  of  the  statocysts 
is  cast  off  and  the  statoliths  or  small  particles  of  sand  they 


32  THE  TROPISMS 

contain  are  consequently  ejected.  The  prawns  replace 
these  by  small  bodies  which  they  pick  up  with  their  chelae  and 
put  in  the  statocyst.  Kreidl  placed  several  Palaemons  after 
moulting  in  a  dish  containing  some  small  bits  of  iron,  which 
the  prawns  put  into  their  statocysts  in  place  of  the  usual 
sand.  If  now  a  strong  magnet  was  brought  near,  the  bits 
of  iron  would  be  brought  toward  it  and  exercise  a  pull 
analagous  to  the  vertical  pull  of  gravity.  Kreidl  found  that 
by  changing  the  position  of  the  magnet  the  Palaemons  could 
be  induced  to  orient  their  bodies  in  any  desired  position. 

When  in  the  web  many  kinds  of  orbweaving  spiders  hang 
with  their  heads  down.  Mosquitoes,  on  the  other  hand, 
when  resting  on  a  vertical  wall  usually  have  the  reverse 
orientation.  Other  insects  when  under  rocks  or  boards 
tend  to  hang  down  from  the  upper  surface  rather  than  rest 
on  the  lower  one.  Lady  beetles,  according  to  Loeb,  when 
placed  in  a  dark  box  uniformly  crawl  upward  and  come  to 
rest  at  the  highest  point.  Cockroaches  prefer  to  rest  on  the 
vertical  sides  of  a  box  rather  than  the  top  or  bottom,  but  they 
do  not  seem  to  have  a  marked  tendency  to  crawl  upward  to 
the  highest  point.  A  tendency  to  crawl  upward  is  not  un- 
common in  insect  larvae,  especially  those  which  feed  on 
plants,  a  trait  which  is  naturally  of  service  in  leading  them 
to  their  food. 

The  tendency  to  maintain  a  certain  orientation  with 
respect  to  gravity  is  more  common  than  the  bent  toward 
upward  or  downward  migration.  This  is  especially  true 
of  higher  animals,  and  among  the  vertebrates  orientation  is 
practically  the  only  response  to  this  force.  The  inner  ear 
in  vertebrates  plays  an  important  part  in  the  maintenance 
of  the  normal  position,  but  a  description  of  the  numerous 
experiments  on  this  subject  would  carry  us  too  far.  The 
presence  of  a  specific  sense  organ  for  the  maintenance  of 


THIGMOTAXIS  OR  STEREOTROPISM  33 

equilibrium  is  by  no  means  general.  Such  organs  occur  in 
medusae,  ctenophores,  turbellaria,  Crustacea,  mollusks  and 
vertebrates,  and  have  been  evolved  along  many  independent 
lines  of  descent.  They  may  be  absent  in  many  species  of 
all  these  groups,  excepting  the  ctenophores  and  vertebrates, 
without  entailing  any  loss  of  the  sense  of  equilibrium. 
Geotropic  irritability  in  many  cases  seems  to  be  quite 
generally  distributed  throughout  the  body.  Where  special 
organs  of  equilibration  have  been  evolved  orientation  is 
generally  only  partially  dependent  upon  them.  In  but 
few  cases  is  orientation  to  gravity  entirely  destroyed  when 
these  organs  are  removed. 

THIGMOTAXIS  OR  STEREOTROPISM 

The  lives  of  all  animals  are  spent  in  more  or  less  per- 
manent contact  with  solid  objects  and  reactions  to  the 
stimuli  thus  afforded  are  universal.  Contact  of  various 
kinds  means  food,  enemies,  shelter  and  many  other  things 
of  interest  to  the  organism  or  its  posterity.  Any  animal  in 
order  to  stand  the  least  chance  of  survival  must  be  endowed 
with  the  power  to  react  to  contact  with  various  solid  ob- 
jects in  an  adaptive  manner.  There  are  such  varied  modes 
of  reaction  to  contact  that  the  Hmitation  of  our  descriptive 
terms  is  a  matter  of  unusual  difficulty.  A  contact  reaction 
by  a  pseudopod  of  an  Amoeba  and  the  heels  of  a  mule  are 
very  different  kinds  of  behavior  although,  on  reflection,  it 
is  evident  that  they  possess  certain  features  in  common. 
One  class  of  reactions  to  contact  has  been  termed  by  Loeb 
stereotropism,  which  he  defines  as  "  the  peculiarity,  possessed 
by  some  animals,  of  orienting  their  bodies  in  a  definite  way 
toward  the  surface  of  other  solid  bodies,"  and  he  distinguishes 
stereotropism  from  other  responses,  such  as  locomotor 
movements,  which  follow  the  application  of  a  contact  stimu- 


34  THE  TROPISMS 

lus.  Verworn  coined  the  word  thigmotaxis,  which  is  practi- 
cally synonymous  with  stereotropism,  in  order  to  designate 

"all  those  cases  of  barotaxis in  which 

the  phenomena  are  caused  by  the  more  or  less  strong  contact 
of  living  substance  with  solid  bodies,"  the  term  barotaxis 
embracing  reactions  which  are  "called  forth  by  pressure 
acting  unequally  on  different  sides"  of  the  body.  Rheotaxis 
caused  by  currents  of  air  or  water,  geotaxis  which  according 
to  Verworn  is  produced  by  differences  in  pressure  on  differ- 
ent parts  and  thigmotaxis  are  all  classed  by  this  writer  as 
special  cases  of  barotaxis. 

Thigmotaxis  may  be  positive  or  negative,  the  difference 
depending  in  many  cases  on  the  strength  of  the  stimulus. 
This  is  shown  by  the  reaction  of  the  rhizopod  Orbitolites. 


^  ^%^^ 


Fig.  3. — Showing   the   positive  reaction  of  Amoeba  to  contact  with  a 
solid  object.     (After  Jennings.) 

When  quiet  this  form  sends  out  long  and  very  fine  filament- 
ous pseudopodia.  When  these  are  brought  in  gentle  con- 
tact with  another  object  the  protoplasm  flows  out  along  the 
filament  and  finally  draws  the  whole  body  toward  the  object. 
If  the  pseudopod  is  cut  or  pressed  upon  by  a  needle  the 
protoplasm  flows  centrally  and  the  pseudopod  is  withdrawn. 
Amoeba  when  floating  in  the  water  usually  sends  long  pseu- 
dopods  out  in  various  directions.  If  one  of  these  strikes 
the  bottom  it  commonly  adheres  to  it;  the  endoplasm 
streams  into  it  from  the  main  body  and  finally  the  whole 
Amoeba  begins  to  crawl  in  the  direction  of  the  attached 


THIGMOTAXIS   OR  STEREOTROPISM  35 

pseudopod.  Such  a  reaction  is  of  evident  service  to  the 
Amoeba  in  leading  it  mto  contact  with  objects  where  it 
secures  food  and  protection.  The  process  of  food  taking 
itself  is  probably  dependent  to  a  certain  extent  upon  a 
thigmotactic  motion.  Strong  contact  such  as  striking  a 
pseudopod  with  a  needle  causes  the  reverse  or  withdrawing 
reaction.  By  repeated  application  of  the  stimulus  to  one 
side  Amoeba  may  be  driven  about  in  any  desired  direction. 

Paramoecia  were  found  by  Jennings  to  react  to  contact 
by  collecting  against  solid  objects.  Often  bits  of  substance 
are  fairly  coated  by  these  infusorians.  The  cilia  in  contact 
with  the  object  cease  beating  and  stand  out  at  right  angles 
to  the  body  while  the  cilia  of  the  oral  groove  and  sometimes 
also  in  other  parts  of  the  body,  continue  their  movement. 
Among  the  hypotrichous  infusoria  the  tendency  to  keep  in 
contact  with  solids  is  very  strong.  In  these  forms  the  cilia 
on  the  imder  side  of  the  body  are  metamorphosed  into  cirri 
which  are  employed  in  running  about  over  objects  much  in 
the  same  way  as  an  insect  uses  its  legs.  An  Oxytricha  is 
described  by  Verworn  as  happening  to  get  upon  the  round 
egg  of  the  clam  Anodonta.  It  ran  about  for  hours  unable 
to  leave  the  surface  since  the  egg  rested  on  the  bottom  at 
only  one  point. 

The  infusorian  Loxophyllum  is  usually  engaged  in  gliding 
about  on  its  right  side.  It  is  apparently  indifferent  as 
regards  orientation  to  gravity,  as  it  will  move  about  on  the 
under  side  of  a  surface  film  with  its  right  side  uppermost 
as  well  as  on  the  bottom.  "WTien  turned  over  on  its  left 
side  it  rights  itself  in  several  different  ways.  Even  small 
pieces  show  the  same  tendency  to  keep  the  right  side  in 
contact  with  solids  and  they  will  also  right  themselves 
when  turned  over.  Much  of  the  righting  movements  of  the 
lower  animals  consists  in  the  effort  to  keep  a  certain  side  in 


36  THE  TROPISMS 

contact  with  a  solid  rather  than  in  any  response  to  gravity. 
This  is  notably  the  case  in  the  fresh-water  Hydra.  In 
Planaria  there  is  an  effort  to  keep  the  ventral  surface  in 
contact  with  some  object  whatever  position  in  relation  to 
gravity  this  may  involve.  Planaria  maculata  shows  a 
modification  of  the  thigmotactic  response  which  Pearl 
has  called  goniotaxis.  When  placed  in  a  dish  these  animals 
form  groups  in  the  angle  between  the  bottom  and  sides. 
The  amount  of  surface  in  contact  with  the  solid  is  not 
appreciably  greater  than  when  the  Planarian  is  on  a  flat 
surface,  but  what  is  sought  by  the  animal  is  a  situation  such 
that  the  body  becomes  bent  at  an  angle. 

Thigmotaxis  is  a  very  common  trait  among  worms  in 
general.  The  effort  to  get  into  holes  or  crevices,  or  to  work 
in  under  rocks,  by  which  these  forms  secure  protection  from 
their  various  enemies,  is  to  a  large  extent  a  manifestation  of 
this  type  of  reaction.  Maxwell  showed  that  if  specimens  of 
Nereis,  which  are  usually  found  in  burrows  in  the  sand 
near  the  seashore,  are  placed  in  a  dish  with  a  number  of 
glass  tubes  just  large  enough  for  them  to  enter,  they  will 
crawl  into  the  tubes  and  remain  there  even  when  exposed 
to  direct  sunlight  which  is  strong  enough  to  kill  them. 

Most  amphipods  and  many  isopods  are  strongly  thigmo- 
tactic and  tend  to  collect  in  crevices  between  rocks  or  among 
masses  of  seaweed  where  they  secure  contact  over  a  consider- 
able surface  of  the  body.  Among  insects  the  instinct  to 
creep  into  crevices  is  a  common  trait.  Earwigs  if  given  an 
opportunity  to  wedge  themselves  under  a  glass  plate  will 
remain  there,  in  spite  of  their  negative  phototaxis,  even 
when  exposed  to  strong  Hght.  The  moth  Amphipyra,  al- 
though positive  in  its  reaction  to  light,  will  cease  its  photo- 
tactic  activities  if  given  an  opportunity  to  crawl  under  a 
glass  plate,  where  it  will  remain  quiet  (Loeb). 


RHEOTAXIS  37 

RHEOTAXIS 

Under  the  head  of  rheotaxis  are  included  the  movements 
of  animals  as  directed  by  currents.  This  response  is  shown 
even  among  very  low  forms.  The  plasmodium  of  the  slime 
mould  Aethalium  when  placed  on  a  filter  paper  along  which 
a  current  of  water  is  passing  slowly  creeps  opposite  the 
direction  of  the  flow.  It  is  not  improbable  that  spermatozoa 
migrate  up  the  oviduct  on  account  of  the  current  which 
the  beating  of  the  cilia  causes  to  flow  toward  the  uterus. 

Among  fishes  orientation  to  currents  is  a  common  phenom- 
enon. Many  fishes  have  the  instinct  to  head  up  stream 
and  swim  against  the  current.  Lyon  has  shown  that  this 
is  due  in  large  measure  to  reactions  to  movements  of  objects 
in  the  visual  field.  In  his  experiments  he  used  an  aquarium 
with  a  glass  bottom  ''so  supported  that  the  bottom  was 
freely  accessible.  Close  along  the  bottom,  beneath  the  glass, 
could  be  drawn  a  long  piece  of  white  cloth  with  black 
strips  painted  across  it.  This  would  give  the  impression  of  a 
moving  bottom.  Fish  (Fundulus)  placed  in  the  aquarium 
oriented  themselves  with  the  head  in  the  direction  of  the 
moving  bottom  and  swam  along  it  to  the  end  of  the  aquarium. 
Reversing  the  movement  of  the  bottom  reversed  the  orien- 
tation and  movement  of  the  fish." 

The  orientation  of  animals  to  currents  has  been  explained 
as  due  to  differences  in  pressure  produced  by  the  current  in 
different  parts  of  the  body.  It  is  obvious  that  such  differences 
can  be  produced  only  when  the  current  moves  past  the  animal. 
Where  the  animal  is  entirely  immersed  and  is  carried  along 
passively  the  effect,  as  Lyon  points  out,  is  the  same  as  if 
it  were  in  quiet  water.  It  is  like  a  man  in  a  balloon  carried 
by  the  wind.  Moving  at  the  same  rate  as  the  air  about  him 
he  becomes  conscious  of  motion  only  when  he  can  see  objects 
on  the  earth  passing  by  beneath. 


38  THE  TROPISMS 

In  another  experiment  by  Lyon  fish  were  placed  in  a 
long  bottle  which  was  corked.  When  the  bottle  was  pulled 
through  the  water  the  fish  immediately  swam  opposite  the 
direction  of  movement.  If  the  bottle  was  allowed  to  flow 
down  stream  the  fish  would  swim  to  the  up  stream  end; 
if  it  was  pulled  up  stream  the  fish  would  swim  to  the  down 
stream  end.  It  is  evident  that  rhfiotaxis  takes  place  through 
orientation  to  objects  in  the  field  of  vision.  In  young 
lobsters  Hadley  has  shown  that  rheotaxis  is  a  sight  response 
much  as  in  fishes.  In  later  stages  when  the  lobsters  keep 
j  closer  to  the  bottom  rheotaxis  is  gradually  lost. 

Blinded  fish  may  orient  themselves  to  currents  so  long  as 
they  are  in  contact  with  the  bottom  of  the  stream,  but  when 
they  leave  the  bottom  they  lose  entirely  their  rheotactic 
response,  unless  different  parts  of  the  stream  with  which 
they  come  in  contact  have  different  velocities. 

Many  insects  tend  to  fly  against  the  wind  (anemotropism). 
This  also  is  probably  a  sight  reflex,  since  the  insect  receives 
pressure  from  the  air  only  when  flying  against  it.  May 
flies,  according  to  Radl,  often  hover  over  one  spot,  slowly 
rising  and  sinking,  but  keeping  their  bodies  facing  the  wind 
and  their  long  fore-legs  stretched  forw^ard.  The  same  trait 
is  shown  by  many  insects,  especially  flies,  even  w^hen  there  is 
no  perceptible  breeze.  Often  a  group  of  flies  will  keep 
hovering  near  one,  and  will  move  as  one's  body  moves  always 
keeping  away  at  about  the  same  distance.  Apparently 
v»'e  have  to  do  here  with  an  effort  to  maintain  a  certain 
relation  to  the  visual  field  which  so  largely  determines  the 
rheotactic  responses  of  fishes. 

It  is  ob\dous  that  under  the  head  of  rheotaxis  phenomena 
have  been  included  which  are  quite  unlike  in  their  causation, 
some  of  them  being  reactions  to  pressure  differences,  others 
presenting  interesting  points  of  similarity  to  phototaxis. 


COMPENSATORY  MOVEMENTS  39 

COMPENSATORY  MOVEMENTS 

A  class  of  phenomena  ha\ing  certain  relations  to  rheotaxis 
are  the  so-called  compensatory  motions  of  animals.  These 
may  easily  be  illustrated  by  a  common  frog.  If  a  frog  be 
slowly  rotated  about  a  vertical  axis  it  will  turn  its  head 
and  may  begin  to  walk  opposite  the  direction  of  movement. 
If  it  is  tilted  downward  in  front,  the  head  will  be  raised, 
while  if  it  is  inclined  upward,  the  head  will  be  lowered. 
Various  combined  motions  will  be  responded  to  by  move- 
ments which  tend  to  keep  the  head  in  the  same  position  as 
before. 

A  pigeon  when  slowly  rotated  on  a  horizontal  turntable 
turns  its  head  opposite  the  direction  of  movement  until 
it  reaches  a  certain  angle  with  the  body  when  it  is  suddenly 
jerked  back  to  its  original  position.  It  immediately  repeats 
the  previous  movement  until  its  head  reaches  again  the 
maximum  angle  when  it  is  jerked  back  again  as  before. 
If  the  head  is  held  during  rotations,  compensatory  motions 
follow^ed  by  regular  jerking  back  movements  are  performed 
by  the  eyes.  Mammals  show  similar  movements,  and  mice 
and  several  other  forms  run  around  on  the  turntable  opposite 
the  direction  of  rotation.  An  interesting  form  of  compen- 
satory movement  is  shown  by  the  common  domestic  fowl. 
Hold  an  individual  in  the  hands  and  move  it  slowly  back 
and  forth,  up  and  down,  or  side  wise.  If  the  fowl  is  not 
carried  too  far  the  head  will  keep  in  almost  exactly  the  same 
position,  the  neck  being  often  stretched  out  to  its  extreme 
length  before  the  head  follows  the  movements  of  the  body. 

It  was  formerly  supposed  that  in  vertebrate  animals 
compensatory  motions  were  dependent  on  the  semicircular 
canals,  but  it  was  found  that  these  motions  still  persisted 
after  the  semicircular  canals  were  plugged  up  or  extirpated, 
or  after  the  nerves  supplying  them  were  cut.     The  otoHths 


40  THE  TROPISMS 

are  not  necessary   organs   for   these   movements    (Lyon). 

In  insects  compensatory  motions  are  easily  observed. 
If  flies,  beetles  and  many  other  forms  are  rotated  on  a  tm-n- 
table  they  generally  walk  around  opposite  the  direction  of 
rotation.  This  reaction  apparently  depends  upon  the  eyes 
since  it  is  no  longer  performed  if  the  eyes  are  blackened  over. 
If  robber  flies  or  dragon  flies  are  held  in  the  hand  and  rotated 
the  head  will  show  reverse  movements  much  as  in  the  frog. 
In  the  crayfish  and  other  higher  crustaceans  compensatory 
motions  are  shown  by  the  eye  stalks.  If  a  specimen  is 
rotated  slowly  about  its  long  axis  the  eye  stalks  will  move 
opposite  the  direction  of  rotation.  Similar  movements 
follow  upon  rotation  about  a  vertical  axis.  Blacking  over 
the  eyes  of  the  crayfish  causes  a  diminution  of  compensatory 
motions  upon  rotation  in  a  vertical  plane,  but  produces 
little  or  no  effect  on  movements  of  the  eye  stalks  upon  rota- 
tion about  a  dorso- ventral  axis  (Lyon). 

In  many  cases  the  effort  to  keep  a  constant  visual  field  is 
an  important  element  in  determining  compensatory  motions, 
but  it  is  not  the  sole  cause  of  such  motions  in  all  forms. 
Sight  is  probably  of  more  importance  than  any  other  single 
factor,  but  static  organs  in  some  cases  seem  to  play  a  part 
also.  In  the  dog-fish,  however,  Lyon  has  shown  that  com- 
pensatory movements  of  the  eyes  regularly  occur,  although 
somewhat  weakened,  in  specimens  in  which  both  the  optic 
and  the  auditory  nerves  have  been  cut. 

PHOTOTAXIS 

Of  the  numerous  papers  on  tropisms  a  greater  number 
have  been  devoted  to  phototaxis  than  to  any  other  subject. 
Only  a  very  brief  discussion  of  the  results  therefore  can  be 
attempted  here.  Investigation  has  shown  that  reactions 
to  light  do  not  fall  under  any  one  general  scheme  of  explana- 


PHOTOTAXIS  41 

tion.  Loeb  early  distinguished  phototaxis  or  heliotropism 
from  mere  reaction  to  differences  in  the  intensity  of  light 
(Unterschiedsempfindlichkeit)  where  there  are  no  definitely 
directed  movements  and  subsequently  appUed  the  term 
photokinesis  to  the  latter  phenomenon.  The  tube-dwelling 
annelid  Serpula  may  be  made  to  suddenly  draw  back  if 
a  shadow  is  thrown  upon  its  expanded  gills.  If  the  light 
be  suddenly  increased  no  reaction  occurs.  In  the  bivalve 
Psammobia,  Nagel  found  that  sudden  increase  of  light  caused 
a  retraction  of  the  extended  siphons;  while  many  other 
species  (Cardium,  Mactra,  Solen)  would  give  a  similar 
reaction  to  shadows.  In  this  connection  may  be  mentioned 
the  reactions  to  shadows  of  the  large  leech  Qepsine  which 
is  parasitic  on  turtles.  If  a  shadow  is  thrown  upon  a  lot 
of  hungry  leeches  in  a  dish  of  water  they  will  raise  up  and 
extend  the  anterior  part  of  the  body  and  sway  it  about  in 
various  directions.  The  function  of  this  response  is  to  enable 
the  leeches  to  attach  themselves  to  any  passing  turtle  and 
thereby  secure  their  food. 

Animals  may  form  collections  in  shaded  localities  not 
because  they  are  negatively  oriented  by  the  rays  of  light, 
but  because  light  stimulates  them  to  general  activity,  and 
when  they  happen  to  come  into  a  place  where  they  are  less 
stimulated  they  become  relatively  quiet.  Loeb  found  that 
if  fresh  w^ater  planarians  are  placed  in  a  round  dish  in  front 
of  a  window  they  collect  at  the  sides  of  the  dish  which  are 
more  or  less  shaded  instead  of  at  the  side  farthest  away  from 
the  window  where  they  would  naturally  assemble  if  they  were 
guided  solely  by  negative  phototaxis.  In  bright  light  these 
animals  are  active  and  when  they  wander  into  a  shaded 
spot  they  move  more  slowly  and  thus  tend  to  collect  there, 
Parker,  however,  has  shown  that  Planaria  maculata  is 
oriented  to  a  certain  extent  by  the  rays  of  light,  and  Walter's 


42  THE  TROPISMS 

extended  studies  prove  that  both  phototaxis  and  photokine- 
sis  are  factors  which,  in  varying  degrees,  determine  the 
behavior  of  this  species. 

Positive  phototaxis  is  often  combined  with  photokinesis. 
In  the  large  amphipod  Talorchestia  there  is  a  marked  photo- 
taxis. Specimens  placed  in  a  glass  dish  before  the  window 
or  an  artificial  light  keep  hopping  toward  the  light  and  strug- 
gling to  get  as  near  it  as  possible  for  hours  at  a  time.  Yet  if 
individuals  chance  to  get  into  a  shaded  region  where  they  are 
less  strongly  stimulated  they  often  remain  there.  Among 
positively  phototactic  insects  similar  behavior  is  not  uncom- 
mon. Many  insects  are  quite  spasmodic  in  their  photo- 
taxis. They  may  run  about  in  apparent  unawareness  of 
light  and  then  suddenly  become  seized  with  an  impulse  to 
go  toward  it.  These  insects  frequently  keep  in  shaded 
localities  most  of  the  time  and  would  ordinarily  be  thought 
to  be  negatively  phototactic  while  in  reality  they  only 
manifest  a  tendency  to  rest  in  shaded  places  into  which 
they  happen  to  wander.  The  proclivity  to  crawl  under 
objects  is  commonly  also,  in  part  at  least,  a  manifestation  of 
positive  thigmotaxis. 

Among  animals  which  regularly  go  toward  or  away 
from  the  light  there  are  considerable  variations  in  the  method 
employed.  WTiile  in  many  forms  there  is  a  direct  orientation 
to  the  rays,  in  others  the  orientation  is  only  brought  about 
indirectly.  Larv2e  of  blow  flies  commonly  crawl  away 
from  the  light,  but  by  a  method  which  I  have  elsewhere  re- 
ferred to  as  the  selection  of  random  movements.  To  quote 
from  my  previous  account,  "^\Tien  strong  light  is  thrown 
on  a  fly  larva  from  in  front,  the  anterior  end  of  the  creature 
is  drawn  back,  turned  toward  one  side,  and  extended  again. 
Often  the  head  is  moved  back  and  forth  several  times  before 
it  is  set  down.    Then  it  may  set  the  head  down  when  it  is 


PHOTOTAXIS  43 

turned  away  from  the  light  and  pull  the  body  around.  If 
the  head  in  moving  to  and  fro  comes  into  strong  light  it  is 
often  retracted  and  extended  again  in  some  other  direction, 
or  it  may  be  swung  back  without  being  withdrawn.  If  a 
strong  light  is  thrown  upon  a  larva  from  one  side  it  may 
swing  the  head  either  toward  or  away  from  the  light.  If 
the  head  is  swung  toward  the  light,  it  may  be  withdrawn  or 
flexed  in  the  opposite  direction,  or,  more  rarely,  moved  to- 
ward the  hght  still  more.  If  it  is  turned  away  from  the  light 
the  larva  usually  follows  up  the  movement  by  locomotion. 
Frequently  the  lai'va  deviates  considerably  from  a  straight 
path,  but  as  it  continually  throws  the  anterior  end  of  the 
body  about  and  most  frequently  follows  up  the  movement 
which  brings  it  away  from  the  stimulus,  its  general  direction 
of  locomotion  is  away  from  the  light.  In  very  strong  illum- 
ination the  extension  of  the  anterior  part  of  the  body  away 
from  the  light  is  followed  by  a  retraction,  since  in  whatever 
direction  it  may  extend  it  receives  a  strong  stimulus  and  the 
larva  wTithes  about  helplessly  for  some  time.  Sooner  or 
later,  however,  it  follows  up  the  right  movements." 

The  earthworm  was  found  to  orient  itself  in  a  similar 
manner.  "  If  a  strong  light  is  held  in  front  of  the  worm  it  at 
first  responds  by  a  vigorous  contraction  of  the  anterior  part 
of  the  body;  it  then  swings  the  head  from  side  to  side,  or 
draws  it  back  and  forth  several  times,  and  extends  again. 
If  in  doing  so  it  encounters  a  strong  stimulus  from  the  light 
a  second  time  it  draws  back  and  tries  once  more.  If  it 
turns  away  from  the  light  and  then  extends  the  head,  it 
may  follow  this  up  by  the  regular  movements  of  loco- 
motion. As  the  worm  extends  the  head  in  crawling  it 
moves  about  from  side  to  side,  and  if  it  happens  to  turn  it 
toward  the  light  it  usually  withdraws  it  and  bends  in  a 
different  direction.     If  it  bends  away  from  the  hght  and 


44  THE  TROPISMS 

extends,  movements  of  locomotion  follow  which  bring  the 
animal  farther  away  from  the  som-ce  of  stimulus." 

While  orientation  in  the  earthworm  is  ordinarily  brought 
about  mainly  by  the  selection  of  random  movements,  light 
has  a  certain  power  to  cause  the  worm  to  turn  directly  away 
from  it,  but  this  is  only  a  minor  factor  under  usual  conditions. 
Harper  has  shown  that  if  the  earthworm  PerichoBta  her- 
mudensis  is  exposed  to  very  strong  light  it  turns  directly 
away  from  it,  while  in  weaker  Hght  orientation  is  brought 
about  by  the  method  just  described.  In  the  leeches  Clepsine 
and  NepheHs  negative  orientation  is  effected  in  part  directly, 
and  in  part  by  following  up  those  random  movements 
which  bring  respite  from  the  stimulus. 

Among  the  protozoa  the  light  reactions  of  Stentor  cendeus 
have  been  found  to  occur  in  much  the  same  way  as  the  reactions 
of  ParamcBcium  and  other  infusoria  to  chemicals.  Accord- 
ing to  Mast  the  anterior  end  of  Stentor  is  much  more  sen- 
sitive to  photic  stimuli  than  other  parts  of  the  body.  If  a 
a  Stentor  swims  into  a  more  highly  illuminated  region  if  gives 
the  avoiding  reaction,  swimming  back,  turning  to  the  aboral 
side  and  going  ahead  in  a  new  direction.  If  light  is  coming 
into  a  dish  from  one  side  the  Stentor  gradually  gets  oriented 
to  the  direction  of  the  rays.  If  it  turns  toward  the  light 
it  gives  the  avoiding  reaction  and  keeps  on  repeating  the  action 
until  it  becomes  pointed  away  from  the  light,  when  it  swims 
away  in  a  fairly  direct  path.  The  infusorian  could  not  be 
seen  to  turn  directly  away  from  the  light;  it  swims  away  from 
the  light  because  if  it  starts  to  swim  in  any  other  direction 
its  course  is  checked  and  its  direction  of  swimming  changed. 

Euglena  vindis,  like  Stentor,  was  found  by  Jennings  to 
give  a  motor  reflex  upon  strong  illumination.  The  anterior 
end  of  the  body  which  contains  the  red  eye-spot  is  more 
sensitive  than  other  regions  and  if  Euglena  passes  into  a 


PHOTOTAXIS 


45 


region  of  higher  light  intensity  it  is  apt  to  reverse  its  move- 
ment and  change  its  course.  In  this  way  it  may  avoid 
regions  of  strong  illumination.  In  weak  light,  however, 
Euglena  is  positively  phototactic.  It  swims  in  a  spiral, 
but  in  a  fairly  direct  path  toward  the  light. 
If  during  its  progress  the  light  is  carried  to 
one  side  Euglena  gradually  turns  until  it  is 
oriented  to  the  direction  of  the  rays.  Al- 
though Jennings  attempts  to  explain  the 
orientation  as  a  result  of  a  modified  motor 
reflex,  I  cannot  see  but  that  the  method  em- 
ployed is  one  of  direct  orientation.  Depar- 
tures from  the  line  are  corrected,  not  as  in 
the  earthworm,  by  a  lot  of  undirected  move- 
ments until  the  right  one  is  hit  upon,  but  by 
an  appropriate  turn  in  the  right  direction. 

Among  a  very  large  number  or  organisms 
in  various  phyla  of  the  animal  kingdom  we 
find  that  there  is  a  fairly  definite  and  direct 
orientation  to  the  rays  of  light.  Deviations 
from  the  path  to  or  from  the  light  are  checked 
by  a  movement  which  brings  the  animal  into 
line  again.  In  lower  forms  this  movement 
is  doubtless  an  involuntary  one  based  upon 
the  property  of  responding  to  a  localized 
light  stimulus  by  a  direct  reflex.  Depar- 
ture from  the  line  of  orientation  subjects 
the  animal  to  unequal  stimulation  on  the  two  sides  and 
the  unequal  motor  activity  thus  produced  brings  the  animal 
back  into  line  again  so  that  both  sides  are  equally  stimulated 
and  locomotion  takes  place  either  toward  or  away  from  the 
light  in  the  direction  of  the  rays.  The  course  of  the  animal  is, 
as  it  were,  automatically  regulated.    The  mechanics  of  the 


B: 


Fig.  4. — Euglena 
viridis. 


46  THE  TROPISMS 

process  varies  greatly  in  different  animals.  Orientation  may 
be  effected  by  the  action  of  the  cilia,  flagella,  muscles  of  the 
body  wall,  legs  and  wings;  and  the  way  in  which  special 
parts  function  to  bring  about  orientation  may  be  funda- 
mentally different  in  different  forms.  In  some  cases  the 
whole  body  is  sensitive  to  light,  in  other  cases  only  certain 
regions,  and  in  many  forms  the  response  is  entirely  depend- 
ent upon  the  eyes. 

There  has  been  more  or  less  controversy  as  to  whether 
the  phototactic  reaction  is  due  to  the  direction  of  the  rays 
or  to  the  differences  in  light  intensity.  A  good  part  of  this 
discussion  has  been  based  on  misunderstandings.  If  an 
animal  is  oblique  to  the  rays  of  light  it  naturally  becomes 
stimulated  with  unequal  degree  of  intensity  on  its  two  sides, 
which  probably  causes  the  orienting  movements.  There  is 
little  doubt  that  direction  of  light  in  the  majority  of  cases, 
if  not  in  all,  causes  orientation  in  this  manner;  it  is  possible, 
however,  that  in  more  or  less  transparent  animals  the  direc- 
tion the  rays  take  through  the  tissues  may  determine 
orientation  in  a  way  analogous  to  the  directive  effect  of  the 
galvanic  current,  but  at  present  we  have  no  convincing  evi- 
dence that  such  is  the  case. 

It  was  shown  by  Loeb  in  all  the  animals  he  subjected  to 
the  experiment  that  it  is  the  rays  toward  the  violet  end  of  the 
spectrum  that  are  the  most  potent  in  producing  the  photo- 
tactic  response.  Red  light  has  comparatively  little  orienting 
power  and  many  phototactic  animals  react  to  it  as  to  dark- 
ness. I  have  found,  however,  that  the  amphipod  Orchestia 
agilis  when  brought  into  a  phototactic  dark  room  illumin- 
ated by  light  through  ordinary  red  glass  is  markedly  positive. 
Specimens  in  a  dark  box  held  so  as  to  get  the  red  light  re- 
flected from  one  wall  still  showed  a  positive  response  although 
the  light  was  so  dim  I  could  scarcely  see  their  movements. 


PHOTOTAXIS  47 

WTiile  some  exceptions  to  the  above  rule  have  been  pointed 
out  by  Minkiewicz  and  Bohn,  it  is  one  of  very  general 
validity  and  affords  a  striking  paralleUsm  to  the  phototactic 
movements  of  plants. 

The  phototactic  response  in  animals  may  be  modified  or 
reversed  by  a  variety  of  agents.  Whether  animals  are^ 
positive  or  negative  often  depends  upon  the  intensity  of 
the  light.  This  is  well  illustrated  by  the  reactions  of  Volvox. 
This  form  consists  of  an  almost  spherical  colony  of  cells 
each  of  which  is  furnished  with  a  pair  of  flagella  which  serve 
as  organs  of  locomotion.  One  axis  of  the  colony  is  some- 
what longer  than  the  others  and  upon  this  axis  Volvox 
commonly  rotates  while  swimming.  AMien  exposed  to  a 
moderate  Hght  the  colony  swims  toward  it  in  a  nearly 
straight  line;  when  it  happens  to  get  out  of  orientation  it 
turns  directly  back  into  line  again.  If  it  reaches  a  point  of 
too  great  intensity  its  movements  become  slower,  its  orien- 
tation less  precise  and  it  may  stop  or  swim  about  slowly  in 
various  directions.  If  exposed  to  hght  above  the  optimum 
it  orients  itself  in  the  reverse  direction  and  swims  away. 
Many  flagellate  protozoa  and  the  swarm  spores  of  many 
algse  are  similarly  positive  in  weak  light  and  negative  in 
strong.  Among  higher  organisms  reversals  following  change 
of  light  intensity  are  less  common  although  there  are  several 
cases.  The  earthworm  Allolobophora  joetida  which  is  com- 
monly negative  shows  a  slight  positive  reaction  in  exceed- 
ingly weak  light  (Adams).  A  great  many  positive  forms 
remain  positive  in  as  strong  light  as  has  been  brought  to 
bear  upon  them  and  most  negative  species  remain  negative 
in  as  weak  light  as  they  respond  to  in  any  definite  way. 

The  sense  of  the  phototactic  response  is  sometimes 
changed  by  exposure  to  light  or  darkness.  Groom  and 
Loeb  found  that  the  larvae  of  Balanus  are  positive  in  the 


48  THE  TROPISMS 

morning  even  to  strong  light,  but  later  in  the  day  they  be- 
come negative  in  light  of  less  intensity.  This  apparently 
determines  the  periodic  depth  migration  of  these  forms,  for 
they  are  usually  found  at  the  surface  of  the  sea  in  the  morning 
and  in  the  afternoon  in  deeper  water.  If  a  culture  is  placed 
at  any  time  in  darkness  for  some  hours,  the  larvse  when 
first  exposed  to  light  are  positive,  but  later  become  negative, 
the  change  taking  place  more  quickly  the  more  intense  the 
light.  Similar  effects  of  exposure  have  been  found  in  other 
forms.  A  curious  instance  of  the  effect  of  previous  exposure 
is  afforded,  however,  by  the  amphipod  Orchestia  agilis. 
If  specimens  are  subjected  to  strong  light  they  are  markedly 
positive  and  remain  so  for  a  long  time.  If  they  are  suddenly 
transferred  to  light  of  weaker  intensity  they  immediately 
show  a  decided  negative  reaction.  This,  however,  is  only 
temporary;  within  a  few  minutes  or  half  an  hour  they  all 
become  positive  again.  If  now  they  are  exposed  to  still 
feebler  light  they  again  become  negative.  I  have  performed 
this  experiment  repeatedly  and  have  found  that  these 
changes  occur  in  the  most  decided  and  striking  manner. 

In  some  cases  positive  phototaxis  may  be  actually  in- 
creased, up  to  a  certain  hmit,  with  the  length  of  the  exposure. 
Animals  whose  first  responses  may  be  hesitating  or  indefinite 
get,  as  it  were,  "warmed  up"  to  the  work,  and  finally  be- 
come almost  violent  in  their  efforts  to  reach  the  light.  The 
water  scorpion  Ranatra  after  having  been  kept  in  the  dark 
for  several  hours  is  commonly  negative;  then  it  shows 
spasmodic  fits  of  a  positive  response  which  grow  longer  and 
stronger  until  the  creature  chases  wildly  after  the  light 
and  becomes  wrought  up  into  the  highest  pitch  of  excitement. 
I  have  found  that  fiddler  crabs,  which  when  first  exposed 
to  artificial  light  show  signs  of  alarm  and  run  away,  gradually 
become  more  and  more  strongly  positive  and  after  a  time 


PHOTOTAXIS  49 

become  apparently  oblivious  to  any  other  stimulus  except 
the  light  which  they  slavishly  follow. 

The  effect  of  temperature  on  phototaxis  is  by  no  means 
uniform.  Loeb  found  that  certain  marine  copepods  and  the 
larvse  of  Polygordius  which  are  positively  phototactic  may 
be  rendered  negative  when  subjected  to  a  higher  tempera- 
ture. Strasburger  found,  on  the  other  hand,  that  the 
swarm  spores  of  many  algse  become  positive  at  a  higher 
temperature  and  negative  at  a  lower.  The  flagellate  Chro- 
mulina,  according  to  Massart,  is  positive  at20°C.  but  negative 
at  5°  C.  Orchestia  agilis  has  been  found  by  the  writer  to 
become  strongly  negative  if  dipped  into  water,  but  if  the 
w^ater  is  heated  nearly  to  the  point  of  producing  death  the 
reaction  becomes  positive. 

The  concentration  and  chemical  nature  of  the  mediimi 
also  influence  phototaxis.  It  was  found  by  Loeb  that 
negative  specimens  of  Polygordius  and  certain  copepods  were 
rendered  positive  by  increasing  the  salt  content  of  the  sea 
water,  while  the  addition  of  fresh  water  rendered  specimens 
negative  which  previously  showed  a  positive  response. 
Larvse  of  Palsemonetes  which  are  normally  positive  become 
negative  if  the  sea  water  is  diluted  with  half  its  volume  of 
distilled  water  (Lyon). 

Certain  infusoria,  Stentor  viridis  and  Paramcedum  bur- 
saria,  which  contain  chlorophyll  go  toward  the  light  only 
when  the  supply  of  oxygen  is  insufficient  (Engelmann), 
but  whether  the  response  is  phototactic  or  photopathic  is 
uncertain;  at  all  events  it  seems  to  be  an  adaptation  to 
lack  of  oxygen,  for  in  the  light  oxygen  becomes  produced  as 
in  plants  by  the  chlorophyll  in  the  organism.  The  amphipod 
Jassa  which  is  usually  negative  becomes  markedly  positive 
in  foul  sea  w^ater.  Carbon  dioxide  and  other  acids  were  found 
bv  Loeb  to  cause  positive  phototaxis  in  Gammarus  and  Cy- 


50  THE  TROPISMS 

clops.  In  the  latter  a  weakly  alkaline  condition  seems  to 
induce  a  negative  reaction.  The  experiments  of  Mr. 
Jackson  on  Hyalella,  a  fresh  water  amphipod  which  is 
ordinarily  negative  in  its  response,  have  shown  that  a  variety 
of  substances  cause  a  positive  reaction.  Mrs.  Michener*  in 
experiments  on  a  number  of  diverse  forms  finds  that  if  the 
response  is  negative  it  may  be  made  positive  by  various 
chemicals,  although  normally  positive  forms  are  rarely  if 
ever  rendered  negative.  A  wide  range  of  chemicals  was 
experimented  with  and  a  positive  response  was  evoked  by 
acids,  salts,  alkalies  and  various  other  substances,  as  in  the 
experiments  of  Mr.  Jackson,  showing  that  there  is  no  definite 
relation  between  the  nature  of  the  response  and  the  class  of 
chemical  compounds  employed  as  stimuli. 

The  effect  of  contact  stimuli  on  phototaxis  affords  one  of 
the  most  curious  phenomena  connected  with  the  modi- 
fication of  the  responses  of  organisms  to  light.  In  many 
cases  contact  causes  an  immediate  reversal  of  the  response. 
Miss  Towle  found  that  negative  specimens  of  the  ostracod 
Cypridopsis  were  rendered  positive  by  being  picked  up  in  a 
pipette  and  dropped  out  again.  Negative  specimens  when 
colliding  with  one  end  of  a  dish  straightway  became  positive 
and  swam  to  the  other  end.  Yerkes  obtained  similar  re- 
sults in  another  ostracod,  Cypris.  Daphnia,  which  is  ordi- 
narily positive,  may  be  made  negative  for  a  very  short  time 
by  repeatedly  picking  it  up  in  a  pipette.  The  copepod  Temora 
longicornisy  which  is  usually  negative,  was  found  to  become 
temporarily  positive  by  being  shaken  (Pouchet).  Positive 
specimens  of  Labidocera,  on  the  other  hand,  may  be  made 
temporarily  negative  by  handling  them  in  a  pipette  (Parker). 

Possibly  alHed  to  these  effects  is  the  curious  reversal  of 
phototaxis  which  occurs  in  certain  terrestrial  amphipods  when 
thrown  into  water.     Having  observed  that  the  terrestrial 

♦Experiments  unpublished. 


PHOTOTAXIS  51 

Amphipoda  are  usually  positively  phototactic  while  the 
aquatic  species  are  negative  it  occurred  to  me  to  try  the  effect 
of  throwing  terrestrial  forms  into  water.  Specimens  of 
Orchestia  agilis  which  were  very  strongly  positive  were 
employed.  As  soon  as  they  were  in  the  water  they  all 
immediately  became  strongly  negative  and  remained  so 
for  days  in  various  intensities  of  light.  That  the  trans- 
formation is  not  due  to  change  of  temperature  was  shown 
by  the  fact  that  the  same  result  was  obtained  whether  the 
water  was  warmer  or  colder  than  the  air,  or  at  the  same 
temperature.  It  is  not  improbable  that  it  is  the  stimulus 
of  contact  afforded  by  the  water  that  caused  the  reversal 
of  the  response. 

In  the  water  scorpion  Ranatra  contact  stimuli  were  found 
to  have  a  very  marked  effect  on  the  insect's  reaction  to  light. 
Handling  these  creatures  throws  them  into  a  death  feint 
which  entirely  inhibits,  for  a  time,  all  phototactic  response. 
It  frequently  happens  that  on  recovery  from  the  feint  there 
is  a  negative  response  which  later  changes  to  positive. 
If  specimens  which  are  swimming  at  the  end  of  a  dish  nearest 
the  light  are  simply  picked  up  by  the  breathing  tube  and 
dropped  back  into  the  water  they  immediately  begin 
swimming  with  equal  vigor  in  the  other  direction.  They 
may  be  caused  to  reverse  their  phototaxis  in  this  way 
repeatedly.  The  same  effect  can  be  produced  if  they  are 
seized  or  stroked  while  under  the  water.  Specimens  which 
are  making  frantic  efforts  to  go  toward  the  light  when  in 
the  air  may  be  caused  to  become  negative  by  simply  dropping 
them  into  water.  When  taken  out  they  usually  show  a 
marked  negative  response  for  some  minutes,  but  later  be- 
come positive. 

The  method  by  which  animals  orient  themselves  to  hght 
naturally  varies  with  their  nervous  and  muscular  organiza- 


52 


THE  TROPISMS 


tion  and  the  nature  of  their  locomotor  organs.  There  are 
comparatively  few  cases  in  which  light  orients  an  animal 
by  causing  directly  a  greater  contraction  of  the  muscles 
of  the  side  most  affected.  The  observations  of  Mast  on  the 
planulse  of  Eudendrium  indicate  that  these  forms  may  be 
oriented  in  this  way.  Eudendrium  planulse  are  cone- 
shaped  organisms  with  the  wide  end  in  front,  and  having 
the  body  covered  by  cilia  by  means  of  which  they  swin 
through  the  water.  According  to  Mast,  *4f  the  ray  direc- 
tion is  but  slightly  changed  after  the  planulse  are  oriented 


Fig.  5. — The  water  scorpion  Ranatra,  showing  the  different  attitudes 
assumed  according  as  the  light  falls  upon  it  from  in  front  or  from  behind. 
The  arrows  indicate  the  direction  of  the  rays. 

they  do  not  turn  directly  toward  the  source  of  light  in  its 
new  position,  but  merely  swing  the  anterior  end  a  little  far- 
ther toward  it  each  time.  In  the  meantime  the  body  grad- 
ually turns  so  as  to  become  oriented  again.  If  however  the 
direction  of  the  rays  is  changed  to  such  an  extent  that  the 
sides  of  the  organism  become  fully  exposed,  they  with  very 
few  exceptions  appear  to  turn  toward  the  light  at  once.  In 
this  process  they  swing  the  anterior  end  laterally  until  it 
nearly  if  not  quite  faces  the  source  of  light." 
The  larvae  of  Arenicola  according  to  Lillie  orient  them- 


PHOTOTAXIS  53 

selves  to  light  by  bending  directly  toward  the  more  stimu- 
lated side.  These  larvae  swim  by  means  of  two  bands  of  cilia 
around  the  body,  and  for  a  short  time  after  hatching  are 
markedly  positive,  swimming  toward  the  Hght  in  nearly 
straight  lines.  If  while  the  larvae  are  swimming  toward  the 
Kght  the  position  of  the  light  is  changed  they  bend  toward 
the  light  until  oriented  in  the  direction  of  the  rays.  Certain 
salts  were  found  by  Lillie  to  inhibit  muscular  action  while 
they  did  not  interfere  with  the  action  of  the  cilia.  LarvaB 
in  solutions  of  these  salts  would  continue  swimming,  but  their 
phototaxis  was  entirely  destroyed.  Mast  who  has  studied 
the  orientation  of  the  same  form  finds  that  "  by  using  two 
sources  of  Hght  so  situated  that  the  rays  cross  at  right  angles 
in  the  region  where  the  specimen  is  located,  and  then  alter- 
nately intercepting  the  light  from  each  of  the  two  sources, 
it  can  be  seen  clearly  that  the  larva,  by  muscular  movement, 
turns  its  anterior  end  toward  the  source  of  Hght  directly. 
There  is  no  trial  in  this  process.  It  is  an  asynametrical 
response  to  an  asymmetrical  stimulation." 

light  has  a  certain  orienting  effect  on  Planaria  maculata, 
even  in  specimens  devoid  of  eyes  although  it  is  masked  by  a 
large  amount  of  random  activity.  Planaria  turns  directly 
away  from  strong  mechanical  and  chemical  stimuH,  as  shown 
by  Pearl,  by  lengthening  the  side  stimulated  instead  of  by 
contracting  the  opposite  side.  The  effect  of  the  stimulus 
is  upon  the  muscles  in  the  vicinity  of  the  stimulated  point. 
There  is  no  ventral  nerve  cord  with  its  numerous  cross 
connectives  such  as  we  find  in  annelids  and  there  seems  to  be 
no  mechanism  by  which  an  impulse  set  up  on  one  side  can 
be  transmitted  more  strongly  to  the  muscles  of  the  opposite 
side  of  the  body  than  to  any  other  region.  The  turning  away 
is  therefore  effected  in  planarians  in  a  very  different  way 
than  in  annelids,  probably  by  the  contraction  of  the  dorso- 


54  THE  TROPISMS 

ventral  muscles  near  the  stimulated  point.  It  is  not  im- 
probable that  the  negative  phototaxis  of  an  eyeless  Planaria, 
like  its  turning  away  from  strong  stimuli  in  general,  is  due 
to  the  local  effect  of  light  in  the  region  on  which  it  impinges. 

In  the  earthworm  or  leech  turning  away  from  strong  light 
is  accomplished  in  all  probability  by  the  contraction  of  the 
longitudinal  muscles  on  the  opposite  side  of  the  body,  the 
result  being  in  the  nature  of  a  crossed  reflex  which  is  so  com- 
mon in  animals  with  an  axial  central  nervous  system.  In  the 
crustacean  Eubranchipus  which  usually  swims  on  its  back 
there  is  a  marked  positive  phototaxis  if  the  animals  are  sub- 
jected to  a  rather  small  source  of  light.  In  ordinary  day- 
light before  a  window  they  pay  Httle  heed  to  the  Hght,  but 
if  taken  into  a  dark  room  and  exposed  to  light  from  an 
electric  bulb  they  will  swim  toward  it  and  follow  it  about 
in  any  direction.  If  they  are  obhque  to  the  rays  they 
bend  the  tail  suddenly  toward  the  more  illuminated  side 
one  or  more  times  until  they  become  oriented. 

In  most  Crustacea  as  in  most  insects  orientation  is  affected 
through  the  unequal  action  of  the  appendages  on  the  two 
sides  of  the  body.  In  a  form  which  is  positively  phototactic 
light  entering  one  eye  sets  up  impulses  which  passing  into 
the  brain  and  nerve  cord,  cause,  directly  or  indirectly,  move- 
ments predominantly  of  flexion  of  the  legs  of  the  same  side 
and  of  extension  of  the  appendages  of  the  opposite  side  of 
the  body.  If  this  is  a  sort  of  mechanical  reflex  process  we 
should  expect  that,  in  a  positively  phototactic  form,  if  one 
eye  were  destroyed  or  blackened  over,  the  animal  would 
move  continually  toward  the  normal  side.  This  experi- 
ment of  blackening  over  one  eye  was  tried  by  the  writer  on 
the  large  sand  flea,  Talorchestia,  and  it  was  found  that  the 
specimens  no  longer  went  straight  toward  the  light,  but 
performed  circus  movements  toward  the  side  of  the  un- 


PHOTOTAXIS  55 

blackened  eye.  The  same  experiment  was  tried  on  several 
postively  phototactic  insects  with  the  same  results.  In  the 
small  sand  flea  Orchestia  agilis  which  is  sometimes  positive 
and  sometimes  negative  in  its  reactions  to  light  blackening 
over  one  eye  causes  circus  movements  toward  the  normal 
side  when  it  is  positive  and  in  the  reverse  direction  when 
it  is  negative.  Since  throwing  this  form  into  water  changes 
its  phototaxis  from  positive  to  negative  it  can  be  made  to 
go  around  in  a  circle  in  either  direction  according  to  the 
medium  in  which  it  is  placed.  Similar  circus  movements 
after  one  eye  was  blackened  over  have  been  observed  by 
Parker  in  Vanessa,  by  Hadley  in  the  lobster,  and  by  Rddl 
in  various  kinds  of  arthropods. 

Not  all  phototactic  insects,  however,  perform  circus 
movements  when  blinded  on  one  side.  If  the  honey  bee, 
for  instance,  which  is  strongly  positive,  is  treated  in  this 
way  it  usually  follows  the  light  almost  as  directly  as  when  in 
a  normal  condition.  In  the  water  scorpion  Ranatra  I  have 
found  that  while  in  some  specimens  there  is  a  strong  tendency 
to  perform  circus  movements  toward  the  normal  side, 
others  go  toward  the  light  in  a  direct  line.  In  some  instances 
specimens  which  at  first  performed  circus  movements  came 
after  a  number  of  trials  gradually  to  straighten  their  path 
toward  the  fight  until  finally  they  followed  it  in  a  straight 
line.  Back  swimmers,  Notonecta,  which  at  first  performed 
circus  movements  and  approached  the  light  only  after  much 
wasted  effort,  were  found  to  straighten  their  course  and 
follow  the  fight  as  well  as  if  they  had  use  of  both  eyes.  These 
facts  indicate  that  phototaxis  may  fall  to  a  certain  extent 
under  the  pleasure-pain  type  of  beha^dor  which  wdU  be  con- 
sidered in  a  later  chapter.  Light  in  some  animals  is  foUowed 
much  as  an  object  of  interest  is  pursued  by  a  higher  animal. 
If  a  creature  learns  to  go  to  the  light  an  element  of  satisfac- 


56  THE  TROPISMS 

tion  or  pleasure  is  probably  associated  with  the  phototactic 
response. 

There  are  a  few  cases  in  which  positively  phototactic 
species  swim  backward  instead  of  forward  toward  the  light. 
An  instance  of  this  kind  was  found  by  Lyon  in  Palsemonetes. 
Hadley  found  that  larval  lobsters  swim  with  the  head  pointed 
away  from  the  light  whether  they  are  postive  or  negative 
in  their  reaction.  In  the  pycnogonid  Anoplodactylus, 
Cole  observed  that  in  crawling  toward  the  light  the  anterior 
end  was  in  advance,  but  in  swimming  toward  the  light  the 
animals  moved  approximately  backward. 

Fiddler  crabs  form  an  exception  to  most  phototactic 
animals  in  that  in  going  toward  the  light  the  body  is  oriented 
sidewise  instead  of  with  its  longitudinal  axis  in  the  direction 
of  the  rays.  These  animals  are  strongly  positive  and,  like 
Ranatra,  their  response  becomes  stronger  with  longer  ex- 
posure. They  are  timid  animals  and  often  a  sudden  move- 
ment of  the  light  will  send  them  scuttling  away  in  alarm, 
but  after  following  the  light  for  some  time  they  become 
more  oblivious  to  other  stimuli  and  slavishy  follow  it  with 
less  show  of  fear.  Orienting  movements  are  different  from 
those  used  in  ordinary  locomotion  which  is  not  the  case  with 
most  forms.  Other  animals  may  orient  themselves  by  walk- 
ing faster,  so  to  speak,  on  one  side  than  on  the  other,  but 
with  lateral  orientation  special  movements  have  to  be  per- 
formed in  order  to  change  the  direction  of  locomotion. 
Here  again  it  is  difficult  to  apply  the  theory  of  forced 
orientation  in  its  usual  form,  and  we  are  led  to  conclude  that 
the  reaction  to  hght  is  to  a  certain  extent  one  of  the  pleasure- 
pain  type. 

According  to  Rddl  phototaxis  has  an  intimate  relation  to 
vision.  In  a  number  of  experiments  Cole  has  found  that 
reactions  to  light  are  influenced  by  the  extent  to  which  the 


PHOTOTAXIS  57 

eyes  are  capable  of  forming  images.  If  animals  are  stimu- 
lated by  two  sources  of  light  of  the  same  degree  of  intensity 
but  of  different  area,  the  forms  without  eyes  or  in  which  the 
eyes  produce  no  image  turn  practically  as  often  to  the  one 
source  of  light  as  the  other  while  animals  with  eyes  producing 
a  distinct  image  will,  if  positively  phototactic,  generally 
turn  toward  the  light  of  larger  area.  With  the  perfection 
of  the  organs  of  vision  the  primitive  phototactic  tenden- 
cies of  animals  may  become  modified  so  as  to  afford  the 
basis  for  the  reactions  to  special  objects  of  the  visual  field 
which  we  find  in  the  more  highly  developed  instincts. 

In  many  animals  there  is  a  strong  reflex  tendency  to  keep, 
so  to  speak,  in  statu  quo  with  the  visual  field.  This  tendency 
accounts  for  many  cases  of  so-called  rheotropism  as  is  shown 
by  the  experiments  of  Hadley  on  the  lobster  and  Lyon  on 
fishes.  The  same  tendency  is,  as  we  have  seen,  manifested 
also  in  compensatory  motions.  Radl  has  shown  that  in 
Daphnia  and  some  other  forms  in  which  the  eyes  are  mov- 
able there  is  an  effort  on  the  part  of  the  eyes  to  become 
oriented  to  the  rays  of  light.  In  the  vertebrates  in  which 
the  eyes  are  freely  movable  the  same  tendency  is  more  or 
less  pronounced.  This  trait  may  be  connected  with  the 
involuntary  tendency  of  animals  to  follow  the  movements 
of  objects  which  cross  their  field  of  vision.  Vision  in  the 
lower  animals  is  concerned  much  more  with  the  movements 
than  with  the  form  of  objects.  It  may  be  possible  to  trace 
more  accurately  than  has  been  done  the  relation  between 
phototaxis,  compensatory  motions  of  the  head  and  body, 
and  the  involuntary  tendency  to  follow  moving  objects 
with  the  eyes.  This  field  of  investigation  has  been  but 
little  explored,  but  it  is  one  which  promises  to  be  fruitful 
of  significant  results. 


68 


THE  TROPISMS 


THERMOTAXIS 

Almost  all  organisms  which  are  free  to  move  go  awa}''  from 
regions  in  which  the  temperature  is  injuriously  high,  and 
many,  although  a  less  number,  withdraw  from  regions  which 
are  colder  than  a  certain  optimum.  If  the  water  near  an 
Amoeba  is  heated  by  a  hot  needle  the  animal  will  contract 
on  the  side  nearest  the  needle,  send  out  pseudopods  in  some 
other  direction  and  crawl  away.  Paramoecium  and  other 
infusoria  form  groups  in  a  region  of  optimum  temperature  by 
the  same  method  employed  in  reacting  to  chemicals.  When 
a  Paramoecium  swims  into  a  region  above  the  optimum  it 
gives  the  motor  reflex  and  goes  in  another  direction.  When  it 
encounters  a  region  below  a  certain  temperature  it  behaves 


Fig.  6. — Reactions  of  Paramoecia  to  heat  and  cold.  One  end  of  the 
slide  is  heated  to  35°  C.  while  the  other  end  is  kept  on  ice.  The  Par- 
amoecia gather  in  an  intermediate  zone  d  c. 


in  the  same  way.  If  Paramoecia  are  placed  in  a  trough  one 
end  of  which  is  heated  to  35°  C.  while  the  other  is  placed  upon 
ice  the  infusorians  will  form  a  band  near  the  middle  where 
the  temperature  ranges  from  24°  to  28°  C.  Previous  ex- 
posure to  higher  or  lower  temperatures  than  those  usually 
experienced  changes  the  optimum  to  a  considerable  degree. 
Most  organisms  above  the  protozoa  turn  directly  away 
from  hot  or  cold  objects.  Sometimes  the  animal  may  escape 
undue  stimulation  by  making  a  number  of  random  move- 


ELECTROTAXIS  59 

ments,  trusting  to  luck  to  get  it  into  a  more  favorable  situa- 
tion. A  comparatively  large  element  of  random  movement 
may  be  combined  with  a  tendency  to  turn  directly  away 
from  the  stimulus,  as  is  apparently  the  case  in  planarians. 
The  subject  of  orientation  to  heat  rays  has  been  little 
studied.  With  aquatic  organisms  it  would  be  difficult 
to  separate  the  reaction  to  heat  rays  and  reaction  to  warmer 
or  colder  regions  of  the  water.  Of  the  effect  of  radiant 
heat  alone  on  the  movements  of  terrestrial  organisms  we 
have  as  yet  very  inadequate  knowledge. 

ELECTROTAXIS 

Reactions  to  the  electric  current  form  no  part  of  the  be- 
havior of  animals  under  natural  conditions.  Nevertheless, 
many  forms  respond  to  the  electric  current  in  a  very  precise 
manner.  An  attempt  to  review  the  rather  extensive 
literature  on  this  subject  would  lead  to  a  considerable 
amount  of  technical  discussion,  and  an  adequate  treatment 
would  require  much  more  space  than  can  be  devoted 
to  it  here.  The  subject  is  one,  however,  which  has  an 
important  bearing  on  the  general  theory  of  tropisms,  and 
we  shall  therefore  describe  briefly  some  typical  cases  of 
electrotactic  response. 

An  Amceha  proteus  placed  in  the  pathway  of  a  weak  electric 
current  assumes  an  elongated  form  and  creeps  toward  the 
cathode.  The  protoplasm  toward  the  end  nearest  the  anode 
apparently  contracts  and  vdih  a  stronger  current  may 
undergo  a  granular  disintegration.  Pseudopods  are  put  out 
on  the  side  toward  the  cathode  and  cause  the  flow  of  the 
protoplasm  in  that  direction. 

When  Paramoecium  is  subjected  to  the  action  of  the  cur- 
rent it  s^dms  with  a  fairly  accurate  orientation  of  the  body 
toward  the  cathode.    If  two  poles  of  a  galvanic  battery  are 


60  THE  TROPISMS 

inserted  in  the  two  sides  of  a  drop  of  water  containing 
Paramoecia  the  infusorians  will  soon  all  collect  around  the 
negative  pole.  In  solutions  of  sodium  chloride  and  certain 
other  salts  Paramoecium  frequently  swims  backward 
toward  the  anode,  and  Bancroft  has  found  that  true  anodal 
electrotaxis,  in  which  the  infusorians  swim  forward  toward 
the  anode,  may  occur  for  a  short  time  in  solutions  of  sodium 
chloride,  sodium  carbonate  and  other  salts  if  the  Para- 
moecia are  washed  in  distilled  water  before  being  placed  in 
the  solutions. 

Electrotaxis  has  been  observed  in  coelenterates,  worms, 
moUusks,  Crustacea,  the  larvae  of  insects  and  in  fishes,  tad- 
poles and  salamanders.  In  general  the  reaction  of  worms 
andTmollusks  is  negative  and  the  reaction  of  Crustacea 
positive.  In  tadpoles  which  orient  their  bodies  parallel 
with  the  direction  of  the  current  the  direction  of  orientation 
is  dependent  upon  the  strength  of  the  current. 

Orientation  to  the  electric  current  is  apparently  brought 
about  by  the  polar  effect  of  the  current  upon  the  tissues  of 
the  body.  In  Paramoecium  there  is  a  direct  orientation  pro- 
duced by  the  unequal  beat  of  the  cilia  on  the  two  sides.  In 
cathodal  orientation  the  forward  phase  of  the  stroke  of  the 
cilia  is  accentuated  where  the  current  leaves  the  body  so  that 
there  may  be  a  reversal  of  the  effective  stroke;  where  the 
current  enters,  the  backward  stroke  continues  so  the  organism 
naturally  swings  into  line  with  the  current.  In  higher  organ- 
isms there  is  a  distinctly  evident  polar  effect  upon  the 
musculature  of  the  body  which  can  be  seen  to  produce  the 
orientation  of  the  organism.  The  effect  of  the  electric 
current  on  animals  generally  is  to  produce  movements 
which  are  in  a  certain  sense  "forced,^'  and  the  orientation 
so  brought  about  affords  an  excellent  example  of  a  typical 
tropism  as  conceived  by  Dr.  Loeb. 


ELECTROTAXIS  61 

BIBLIOGRAPHY 
Bancroft,  F.  W.     The  Control  of  Galvanotropism  in  Paramcecium 

by  Chemical  Substances.     Univ.  of  Calif.  Pubs.  Physiology,  3, 

21,  '06. 
BoHN,  G.     Th^orie    nouvelle  du  phototropisme.     C.  R.  Ac.  Sci. 

Paris,  139,  890,  '04.     La  Naissance  de  I'lntelligence.    Paris,  '09. 
Cole,  L,  J.     Notes  on  the  Habits  of  Pycnogonids,  Biol.  Bull.,  2, 

195,  '01.     An  Experimental  Study  of  the  Image-forming  Power 

of  Various  Types  of  Eyes.    Proc.  Am.  Ac.  Arts  and  Sci.,  42, 

335,  '07. 
Davenport,    C.    B.     Experimental   Morphology.     N.   Y.,  '97-99. 
Harper,  E.  H.    Reactions  to  Light  and  Mechanical  Stimulation  in 

the  Earthworm,  Perichceta  hermiLdensis.     Biol.  Bull.,  10, 17,  '05. 
Holmes,  S.  J.  Phototaxis  in  the  Amphipoda.     Am.  Jour.  Physiol., 

5,  211,  '01.     The  Selection  of  Random  Movements  as  a  Factor 

in  Phototaxis.     Jour.  Comp.  Neur.  Psych.   15,  98,  '05.    The 

Reactions  of  Ranatra  to  Light,  1.  c,  15,  305,  '05.    Phototaxis 

in  Fiddler  Crabs  and  its  Relation  to  Theories  of  Orientation. 

1.  c,  18,  493,  '08. 
Holt,  E.  B.,  and  Lee,  F.  S.     The  Theory  of  Phototactic  Response. 

Am.  Jour.  Physiol.,  4,  460,  '01. 
Jennings,  H.  S.      Contributions  to  the  Study  of  the  Behavior  of 

Lower    Organisms.     Carnegie    Inst.    Pubs.    Washington,   '04. 

Behavior  of  Lower  Organisms.     N.  Y.,  '06. 
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mung   mit   dem   Heliotropismus  der  Pflanzen,   Wtirzburg,  90. 
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of  Living  Matter.     N.  Y.  '06. 
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Jour.  Exp.  ZooL,  3,  359,  '06.     Light  Reactions  in  Lower  Organ- 
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Light  and  the  Behavior  of  Organisms.     N.  Y.,  '11. 
Mendelssohn,  M.  Ueber  den  Thermotropismus  einzelliger  Organis- 

men.    Pfluger's  Archiv,  60,  1,  '95. 
MiNKiEwicz,  C.     Sur  le  chromotropisme  et  son  inversion  artificielle. 

C.  R.  Ac.  Sci.     Paris,  143,  785,  '06. 
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NuEL,  J.  P.     La  Vision.     Paris,  '04. 
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Vanessa  antiopa.     Mark  Anniversary  Volume,  453,  '03. 
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62  THE  TROPISMS 

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Exp.  ZooL,  5,  35  and  1 17,  '07. 


CHAPTER  IV 
THE  BEHAVIOR  OF  PROTOZOA 

"  One  of  the  first  lessons  which  the  study  of  animal  behaviour,  in 
its  organic  aspect,  should  impress  upon  our  minds  is,  that  living  cells 
may  react  to  stimuli  in  a  manner  which  we  perceive  to  be  subservi- 
ent to  a  biological  end,  and  yet  react  without  conscious  purpose- 
that  is  automatically." — C.  Lloyd  Morgan,  Animal  Behaviour. 

The  student  of  the  evolution  of  mind  naturally  looks  with 
interest  to  the  behavior  of  those  organisms  which  lie  nearest 
to  the  root  of  the  tree  of  life.  What  mental  powers  are 
evinced  by  the  lowest  animals,  or  whether,  indeed,  the  lowest 
animals  exhibit  any  mental  powers  at  all  are  questions  of 
fundamental  importance  to  comparative  psychology.  Never- 
theless with  all  the  theoretic  interest  and  importance  attach- 
ing to  the  study  of  the  powers  and  performances  of  these 
low  forms  it  is  somewhat  surprising  that  until  quite  recently 
the  subject  attracted  few  serious  investigators. 

Binet  in  his  book  on  the  Psychic  Life  of  Micro-organisms 
makes  one  of  the  first  thorough-going  attempts  to  estimate 

e  extent  of  the  protozoan  mind.  Among  the  psychic 
operations  which  he  claims  are  manifested  are: 

1.  "Perception  of  the  external  object; 

2.  ''  The  choice  made  between  a  number  of  objects; 

3.  '*The  perception  of  their  position  in  space; 

4.  "  Movements  calculated,  either  to  approach  the  body 
and  seize  it,  or  to  flee  from  it." 

Choice  is  manifested  according  to  Binet  in  the  selection  of 
food.  Many  species  live  on  a  few  kinds  of  food  and  refuse 
others.    This  choice  cannot  be  explained  as  due  to  physical 

63 


64  THE  BEHAVIOR  OF  PROTOZOA 

causes;  "it  is  one  of  the  most  incomprehensible  of  phe- 
nomena; it  is  exceedingly  difficult  to  explain  it  without  resort 
to  anthropomorphism."  Protozoa  not  only  perceive  ex- 
ternal objects  but  ''they  also  indicate,  by  their  movements, 
an  exact  knowledge  of  the  position  occupied  by  these  bodies. 
It  might  be  said  that  they  invariably  possess  a  sense  of 
position  in  space.  The  possession  of  this  sense  is  absolutely 
indispensible  to  them,  for  it  does  not  suffice  them  to  know 
of  the  presence  of  an  exterior  body  in  order  to  approach  it 
and  seize  it;  they  must  furthermore  know  its  position,  so  as 
to  direct  their  movements  accordingly. 

"The  simplest  form  of  a  sense  of  localization  is  met  with  in 
the  Amoeba  which,  when  it  closes  about  a  nutritive  particle, 
always  emits  its  pseudopods  at  precisely  that  part  of  its  body 
where  the  foreign  substance  caused  the  irritation.  The 
most  complicated  instance  of  localization  is  met  with  in  the 
Didinium,  which  we  have  so  often  cited;  the  Didinium  knows 
exactly  the  position  of  the  prey  it  follows,  for  it  takes  aim 
at  the  object  of  its  pursuit  like  a  marksman,  and  trans- 
pierces it  with  its  nettle-like  darts." 

Instinct,  memory,  fear  and  a  certain  degree  of  intelligence 
are  among  the  psychic  endowments  with  which  Binet  credits 
the  protozoa.  A  good  sample  of  his  interpretation  of 
protozoan  behavior  is  the  following:  "The  Bodo  caudatus 
is  a  voracious  Flagellate  possessed  of  extraordinary  audacity; 
it  combines  in  troops  to  attack  animalcula  one  hundred 
times  as  large  as  itself,  as  the  Colpods  for  instance,  which 
are  veritable  giants  when  placed  alongside  of  the  Bodo, 
Like  a  horse  attacked  by  a  pack  of  wolves,  the  Colpod  is 
soon  rendered  powerless;  twenty,  thirty,  forty  Bodos  throw 
themselves  upon  him,  eviscerate  and  devour  him  completely 
(Stein). 

"All  these  facts  are  of  primary  importance  and  interest, 


THE  BEHAVIOR  OF  PROTOZOA       65 

but  it  is  plain  that  their  interpretation  presents  difRculties. 
It  may  be  asked  whether  the  Bodos  combine  designedly  in 
groups  of  ten  or  twenty,  understanding  that  they  are  more 
powerful  when  united  than  when  divided.  But  it  is  more 
probable  that  voluntary  combinations  for  purposes  of  attack 
do  not  take  place  among  these  organisms;  that  would  be  to 
grant  them  a  high  mental  capacity.  We  may  more  readily 
admit  that  the  meeting  of  a  number  of  Bodos  happens  by 
chance;  when  one  of  them  begins  an  attack  upon  a  Colopod, 
the  other  animalcula  lurking  in  the  vicinity  dash  into  the 
combat  to  profit  by  a  favorable  opportunity." 

More  recent  investigations  have  shown  that  the  behavior 
of  protozoa  gives  no  evidence  of  the  high  psychic  develop- 
ment assumed  by  Binet.  There  has  been  a  strong  tendency 
on  the  part  of  certain  investigators  to  explain  the  behavior 
of  these  low  forms  as  due  in  large  measure  to  comparatively 
simple  physical  and  chemical  factors.  Others  contend  that 
the  phenomena  are  much  more  complex  and  at  present  defy 
analysis  into  physical  and  chemical  processes,  while  a  few 
go  further  and  maintain  that  we  must  assume  some  super- 
physical  agency,  a  vital  principle,  or  entelechy  of  some 
sort,  to  explain  the  results. 

Much  attention  has  been  bestowed  upon  the  activities  of 
Amoeba,  which  is  generally  assumed  to  be  one  of  the  most 
primitive  of  the  protozoa.  Amoeba  is  a  jelly-like  organism 
with  a  clear,  outer,  relatively  dense  layer  of  protoplasm,  the 
ectosarc,  surrounding  a  more  fluid,  granular  substance,  the 
endosarc,  which  contains  the  nucleus.  The  Amoeba  com- 
monly moves  by  sending  forth  blunt  processes  or  pseudopodia 
from  the  side  of  the  body;  material  flows  into  the  pseudopod 
which  may  increase  greatly  in  size;  then  other  pseudopods 
are  put  forth  on  the  same  side,  the  posterior  part  of  the  body 
being  pulled,  in  the  meantime,  in  the  direction  in  which  the 


66  THE  BEHAVIOR  OF  PROTOZOA 

pseudopods  are  protruded.  The  details  of  the  movements 
vary  considerably  in  different  species.  In  Amoeba  verrv^ 
cosa  Jennings  describes  the  locomotion  as  '*in  many 
respects  comparable  to  rolling,  the  upper  surface  continually 
passing  forward  and  rolling  under  the  anterior  end  so  as  to 
form  the  under  side/'  This  is  shown  by  placing  in  the 
water  particles  of  soot  some  of  which  adhere  to  the  surface 
of  the  animal;  if  a  single  particle  be  watched  it  will  be  seen 
to  pass  around,  as  the  Amoeba  progresses,  like  an  object  on 
the  surface  of  a  rolling  cylinder.  The  anterior  edge  of  the 
Amoeba  is  thin  and  fiat  and  adheres  closely  to  the  substance 
while  the  thicker  posterior  edge  is  free.  The  movement  is 
like  the  rolling  of  a  contractile  sac  wdth  semifluid  contents. 
Amceha  Umax  has  a  similar  method  of  locomotion  but  has 
few  pseudopods.  Bellinger  who  has  studied  the  locomotion 
of  AmxBba  proteus  describes  a  method  of  locomotion  quite 
different  from  that  found  by  Jennings.  Dellinger  hit  upon 
the  device  of  observing  Amoeba  from  the  side,  by  means 
of  a  horizontal  miscroscope.  The  Amoeba  sends  out  a 
pseudopod  free  into  the  water,  which  is  then  bent  down  and 
attached  to  the  substrate;  the  posterior  end  is  now  raised 
and  pulled  forward;  then  another  pseudopod  is  pushed  out 
and  attached  like  the  first  and  the  body  pulled  forward  again. 
During  the  progress  the  Amoeba  is  attached  only  at  a  few 
points  on  short  improvised  feet  which  are  drawn  in  as  the 
Amoeba  passes  over  them. 

In  a  small  unidentified  species  of  Amoeba  the  method  of 
locomotion,  as  observed  by  the  writer,  differs  from  all  the 
foregoing  ones.  There  are  broad  ectoplasmic  pseudopods 
put  out  at  the  clear  anterior  end,  commonly  first  on  one  side 
and  then  on  the  other;  the  endoplasm  flows  into  them  and 
the  hinder  part  of  the  body  which  contains  the  contractile 
vacuole  is  drawn  forward.    There  is  no  rolling ;  particles  on  the 


THE  BEHAVIOR  OF  PROTOZOA       67 

surface  of  the  hinder  half  of  the  animal  may  undergo  little 
change  of  position  for  a  long  time. 

There  are  various  ways  of  imitating  the  movements  of 
Amoeba  by  drops  of  oil  or  other  fluids  subjected  to  changes 
of  surface  tension.  If  a  drop  of  mercury  is  placed  in  dilute 
nitric  acid  and  a  piece  of  potassium  bichromate  placed  near 
it  the  drop  of  mercury  will  bulge  out  toward  the  bichromate 
and  may  surround  it.  The  bichromate  as  it  diffuses  against 
the  mercury  causes  a  diminution  of  surface  tension  at  the 
region  of  contact.  The  stronger  contraction  of  the  rest  of 
the  surface  film  forces  the  mercury  to  protude  at  the  weakest 
point;  producing  an  out-pushing  resembling  the  pseudopod 
of  the  Amoeba.  It  has  been  contended  that  variations  in 
surface  tension  account  in  great  measure  for  the  movements 
of  Amoeba  and  other  Rhizopods  much  as  in  inorganic  fluids. 
There  is  certainly  a  striking  analogy  between  the  phenomena 
in  the  two  cases,  but  the  studies  of  Jennings  have  shown 
that  explanation  of  the  phenomena  is  not  quite  so  simple. 
Jennings  argues  that  the  currents  in  Amoeba  are  not  like 
those  occurring  in  drops  of  fluid  which  are  changing  the 
surface  tension  because  in  the  Amoeba  there  are  no  lateral 
return  currents  which  are  present  in  the  mo\dng  fluid  and 
hence  the  surface  tension  theory  cannot  account  for  the 
Amoeba's  changes  of  form.  It  must  not  be  forgotten  that 
we  are  dealing  with  substances  of  quite  different  consistency 
one  of  which  has  a  cortical  layer  of  considerable  thickness  and 
density  which  is  entirely  absent  in  the  other,  and  that  the 
behavior  of  internal  currents  may  be  affected  by  this  factor; 
but  aside  from  this,  there  are  many  facts  which  seem  to  in- 
dicate that  the  ectoplasm  behaves  more  like  a  muscular  layer 
than  one  whose  contraction  is  entirely  due  to  the  surface  film. 
Contracting  pseudopods  may  become  wrinkled,  which  could 
not  occur  if  mere  surface  film  contraction  were  responsible 


68  THE  BEHAVIOR  OF  PROTOZOA 

for  the  withdrawal.  Sometimes  pseudopods  move  back  and 
forth  as  if  they  possessed  a  certain  degree  of  flexibility  and 
rigidity.  The  rolling  motion  of  certain  species  offers  another 
difficulty,  so  it  cannot  be  said  that,  at  present,  the  surface 
tension  theory  suffices  to  explain  the  movements  of  the 
organism.  That  it  may  play  a  part  in  the  process  is  not 
improbable.  It  must  not  be  forgotten  that  there  are  many 
sm'faces  within  the  mere  outer  layer,  such  as  alveolar  walls, 
etc.,  where  surface  tension  may  be  a  potent  agent.  In 
forms  a  little  higher  than  Amoeba  we  meet  with  a  more  or 
less  fibrillar  ectoplasm  whose  contraction  occurs  much  as 
in  an  ordinary  muscle  fiber.  It  is  not  improbable  that  the 
fundamental  features  of  contraction  in  the  specialized  ecto- 
plasm of  flagellates  and  infusorians  are  the  same  as  in  the 
unspecialized  ectoplasm  of  the  rhizopods  on  the  one  hand 
and  in  the  more  highly  specialized  muscular  tissue  of  higher 
animals  on  the  other.  The  cause  of  muscular  contraction 
is  one  of  the  most  obscure  problems  of  physiology  and  until 
it  is  solved  we  shall  probably  be  unable  to  explain  the  mech- 
anism of  the  movements  of  the  simplest  animals. 

In  taking  food  Amoeba  has  been  described  as  flowing 
around  an  object  and  engulfing  it  in  its  endoplasm  where  it 
undergoes  digestion.  Surface  tension  has  been  supposed 
to  play  a  part  in  the  process.  A  fine  splinter  of  glass  brought 
against  a  drop  of  water  will  be  quickly  drawn  in  through  the 
contraction  of  the  surface  film.  A  drop  of  chloroform 
may  be  made  to  draw  in  a  glass  splinter  covered  wifh 
shellac;  after  the  shellac  is  dissolved  the  glass  splinter  will 
be  extruded,  through  the  force  of  surface  tension,  as  from  a 
drop  of  mercury,  thus  simulating  both  the  ingestion  of  food 
and  the  defecation  of  the  undigested  residue  (Rhumbler). 
The  observations  of  Jennings  on  food  taking  in  Amceha 
proteus  show  that  the  protoplasm  does  not  flow  around  the 


THE  BEHAVOIR  OF  PROTOZOA  69 

object  as  if  drawn  by  surface  tenison;  a  small  pseudopod  is 
put  out  on  either  side  of  the  food;  these  processes  extend 
and  curve  around  it  until  they  meet,  and  then  the  object 
is  drawn  into  the  endoplasm.  In  Amoeba  proteus  and  limax 
"there  is  no  adherence  between  the  protoplasm  and  the 
food  body,"  although  there  is  adherence  in  Amceba  verrucosa 
whose  ectoplasm  seems  to  be  more  adhesive  to  all  kinds  of 
objects. 

Amoeba  like  higher  animals  may  follow  its  food.  Jennings 
describes  an  Amoeba  attempting  to  engulf  a  spherical  cyst  of 
Euglena.  As  the  Amoeba  came  in  contact  with  it  the  cyst 
rolled  away;  the  Amoeba  followed;  the  cyst  continued  to 
be  pushed  ahead  now  one  way  and  now  another  and  the 
Amoeba  changed  its  course  accordingly.    After  the  cyst  had 


Fio.  7. — Reaction   of  Amoeba  to  a  strong  mechanical  stimulus.     The 
arrows  indicate  the  direction  of  the  currents. 

been  rolled  against  an  obstacle  and  the  Amoeba  was  about 
to  succeed  in  capturing  it,  a  large  infusorian  appeared  on  the 
scene  and  swept  it  away. 

The  reactions  of  Amoeba  to  stimuli  may  be  either  positive 
or  negative.  To  a  strong  mechanical  stimulus  such  as  con- 
tact with  the  point  of  a  needle  Amoeba  reacts  by  crawling 
away.  Should  the  stimulus  be  applied  at  the  anterior  end 
the  stimulated  part  stops,  there  is  a  local  contraction  of  the 
ectoplasm  and  the  granules  begin  to  stream  away  from  the 
point  of  contact.  This  stream  meets  the  stream  proceeding 
in  the  general  direction  of  locomotion  and  the  two  combine 


70  THE  BEHAVIOR  OF  PROTOZOA 

to  form  a  stream  which  issues  in  another  direction.  Stimu- 
lation of  the  posterior  end  causes  the  animal  to  quicken  its 
pace.  Stimulating  a  pseudopod  causes  it  to  be  withdrawn; 
and  by  repeated  stimulation  the  animal  may  be  driven 
about  at  will. 

To  the  weaker  stimuli  which  are  received  by  coming  into 
gentle  contact  with  solid  objects,  Amoeba  often  reacts 
positively.  If,  when  floating  about  in  the  water,  one  pseu- 
dopod comes  in  contact  with  a  solid  it  adheres  to  it;  there 
is  a  flow  of  granules  toward  the  point  of  contact,  from  the 
rest  of  the  body;  this  pseudopod  enlarges,  the  others  contract 
until  most  of  the  body  has  flowed  into  the  attached  pseudo- 
pod, when  the  Amoeba  crawls  along  the  surface  of  the  object. 
The  utility  of  the  negative  response  to  strong  mechanical 
stimuli  is  obvious  since  it  enables  the  organism  to  avoid 
injurious  agencies.  The  positive  reaction  tends  to  keep 
the  Amoeba  in  contact  with  solid  objects  where  most  of  its 
food  is  secured  and  where  it  receives  protection.  Probably 
also  it  plays  a  part  in  determining  the  behavior  toward  food. 

To  injurious  chemicals  Amoeba  reacts  much  as  to  strong 
mechanical  stimuli;  it  creeps  away  from  regions  of  higher 
temperature,  it  reacts  negatively  to  light.  All  these  responses 
are  purposive  in  that  they  are  adapted  to  the  preservation 
of  the  organism.  Simple  as  Amoeba  apparently  is  it  manages 
to  cope  very  effectively  with  the  conditions  of  its  existence. 
One  might  conceivably  construct  a  machine  which  would 
run  of  itself,  gather  the  food  needed  to  supply  the  energy 
used  in  its  workings,  avoid  automatically  contact  with 
obstacles  which  would  impair  its  running,  move  away  from 
regions  too  hot  or  too  cold  for  its  efficient  operation,  protect 
itself  by  producing  coverings  in  unfavorable  situations,  and 
guide  itself  into  the  most  favorable  regions  for  its  mainte- 
nance; but  what  a  wonderfully  complicated  mechanism  it 


THE  BEHAVIOR  OF  PROTOZOA  71 

would  have  to  be!  Yet  a  simple,  apparently  almost  struc- 
tureless mass  of  jelly  does  all  this  and  more.  And  if  our 
mechanism  had  the  property  of  repairing  its  own  injuries  and 
producing  other  pieces  of  mechanism  like  itself,  its  structural 
arrangements  would  be  almost  if  not  quite  beyond  our  power 
to  conceive.  One  cannot,  therefore,  but  look  with  a  feeling  of 
admiration  and  w^onder  at  so  comparatively  simple  a  creature 
as  Amoeba,  which  is  capable  of  performing  so  much. 

But  the  story  does  not  end  even  here.  In  addition  to  aU 
the  adaptive  properties  mentioned  Amoeba  has  the  power 
of  modifying  its  behavior  to  suit  new  conditions.  Toward 
water  different  from  that  in  which  it  is  immersed  Amoeba 
reacts  negatively,  but  after  remaining  in  such  water  for 
some  time  it  resumes  its  usual  activities.  "Wlien  first  ex- 
posed to  bright  light  it  withdraws  its  pseudopods  and  re- 
mains quiet,  but  after  continued  exposure  it  adapts  its  be- 
havior to  the  new  conditions  and  again  becomes  active. 
After  a  short  period  of  starvation  Amoeba  moves  about 
more  actively  than  usual,  whereas  when  it  is  gorged  with 
food  it  becomes  relatively  sluggish.  The  condition  of  the 
animal  determines  its  behavior  in  other  ways  and  the  changes 
of  behavior  are  usually  advantageous  to  the  organism.  The 
behavior  of  Amoeba  is  essentially  like  that  of  higher  animals; 
it  avoids  things  which  are  injurious;  it  seeks  things  which 
are  beneficial  and  it  adapts  its  behavior  to  new  conditions. 
Life  is  very  much  the  same  sort  of  thing  whether  in  an 
Amoeba  or  a  man. 

The  chief  rival  of  Amoeba  in  the  attentions  of  the  com- 
parative psychologist  is  Paramoecium.  This  is  a  cigar- 
shaped  infusorian  rounded  in  front  and  pointed  at  the 
posterior  end,  and  covered  by  a  uniform  coating  of  cilia. 
It  has  an  oblique  oral  groove  leading  posteriorly  into  a  gullet 
into  which  are  swept  small  particles  which  serve  for  food. 


72 


THE  BEHAVIOR  OF  PROTOZOA 


By  the  action  of  its  cilia  Paramoecium  swims  through  the 
water  in  a  spiral  course,  rotating  on  its  long  axis  to  the  left 
and  keeping  the  oral  side  facing  the  center  of  the  spiral. 
At  times  Paramoecium  may  swim  backward,  by  reversing 
the  effective  stroke  of  the  cilia,  although  the  direction  of 
rotation  is  unchanged. 

Paramoecium  is  a  common  organism  in  vegetable  infu- 
sions, where  it  subsists  upon  bac- 
teria which  are  swept  by  cilia  into 
its  gullet.  There  seems  to  be  little 
power  of  choice  as  to  what  sub- 
stances are  taken  in;  particles  of 
India  ink  and  carmine  are  swept  in 
and  swallowed  in  the  same  way  as 
its  normal  food,  and  the  organism 
selects  its  food  only  by  swimming 
elsewhere  when  the  materials  swept 
in  are  unsuitable.  Metalnikow, 
however,  states  that  when  Par- 
amoecia  are  fed  with  carmine  for 
fifteen  days  they  gradually  take 
in  less  of  this  substance  and  finally 
refuse  it.  Metalnikow's  experiments 
were  repeated  by  Schaeffer  who 
failed  to  confirm  these  results  when 

Fig.  8.— Paramoecium  cau-   the   carmine  was  kept  in  a  condi- 
datum.  (After  Kellogg.)      ,.  i     .i     .     •.  ii    i  ti 

tion  such  that  it  could  be  readily 

ingested.  Occasional  individuals,  however,  failed  to  take 
in  any  of  the  substance,  but  examination  showed  that  they 
were  deformed  or  had  recently  divided  so  that  the  oral  ap- 
paratus was  not  capable  of  sweeping  in  food.  After  thirty- 
three  days  the  Paramoecia  failed  to  show  any  appreciable 
diminution  in  the  carmine  ingested. 


THE  BEHAVIOR  OF  PROTOZOA       73 

Paramoecium  is  a  very  restless  organism,  swimming 
actively  much  of  the  time.  Occasionally  it  remains  quiet 
when  in  contact  with  solid  objects.  When  manifesting  its 
so-called  thigmotactic  response  the  cilia  in  contact  with 
the  solid  remain  stiff  and  immobile  as  if  anchoring  the  animal 
to  the  spot,  while  the  cilia  over  the  rest  of  the  body  keep 
moving,  although  with  diminished  vigor.  If  bits  of  cotton 
wool  are  placed  in  the  water  Paramoecia  are  more  apt  to 
come  to  rest,  owing  to  the  greater  opportunity  afforded  of 


Fig.  9. — Successive  stages  of  the  motor  reflex  of  Paramoecium.     (After 

Jennings.) 

securing  contact  stimuli.  This  trait  keeps  the  Paramoecia 
among  bacterial  scums  and  in  other  situations  where  they 
may  obtain  their  food. 

The  principal  feature  of  the  behavior  of  Paramoecia  is 
what  Jennings  has  called  the  "motor-reflex"  or  "avoiding 
reaction.'^  It  consists  of  swimming  backward  by  reversal 
of  the  action  of  the  cilia,  turning  to  the  aboral  side  and  then 
going  ahead  again.  This  is  the  stereotyped  response  which 
Paramoecium  gives  in  essentially  the  same  way  when  en- 
countering almost  any  kind  of  stimulus.  If  stimulated  by 
a  fine  needle  on  the  aboral  side  it  will  back  off  and  turn  to- 
ward instead  of  away  from  the  stimulating  object.    Even 


74  THE  BEHAVIOR  OF  PROTOZOA 

when  the  posterior  end  is  stimulated  the  Paramoecium  may 
swim  back  directly  against  the  needle.  A  slight  stimulus 
at  the  posterior  end  often  causes  the  infusorian  to  accelerate 
its  swimming,  but,  with  this  exception,  the  nature  of  the 
response  seems  to  bear  no  relation  to  the  part  of  the  body 
to  which  the  stimulus  is  applied.  The  anterior  end  of  the 
body  is  the  most  sensitive  region,  although  stimuli  at  that 
point  do  not  call  forth  any  different  kind  of  response,  but 
only  evoke  the  usual  motor  reflex  with  greater  readiness. 

The  motor  reflex  or  avoiding  reaction  has  the  effect  of 
getting  the  animal  away  from  inj  urious  stimulations.  Often, 
however,  at  first  it  may  bring  it  into  a  worse  situation  than 
before;  the  Paramoecium  may  back  into  an  injurious  chem- 
ical, or  turn  toward  a  mechanical  or  other  stimulus  which 
affects  its  aboral  side;  if  so,  the  motor  reflex  is  repeated  until 
the  organism  makes  its  escape  from  the  unfavorable  situa- 
tion. The  method  of  adjustment  may  be  clumsy  and  in- 
direct; had  Paramoecium  the  power  as  higher  organisms 
have,  of  turning  directly  away  from  the  stimulus,  its  reac- 
tions would  probably  be  more  effective;  but  its  restless 
activity  and  quick  movements  ai-e  a  partial  compensation 
for  defects  in  precision  of  response. 

The  motor  reflex  may  be  carried  out  in  varying  degrees  of 
completeness.  The  duration  of  the  backward  swimming 
and  the  amount  of  turning  to  the  aboral  side  are  subject 
to  much  variation.  The  two  phases  of  the  response  may 
be  greatly  prolonged  in  a  solution  of  potassium  iodide.  The 
Paramoecium  swims  backward  for  several  minutes;  then 
spins  around  toward  the  aboral  side  for  a  considerable  time 
and  finally  swims  forward.  In  ordinary  water  the  distance 
the  Paramoecium  swims  backward  is  increased  with  a 
stronger  stimulus  and  the  aboral  rotation  may  continue  until 
the   infusorian   describes   several   complete   circles.     With 


THE  BEHAVIOR  OF  PROTOZOA       75 

slight  stimulations  the  first  phase  of  the  reaction  may  result 
m  a  momentary  stopping  of  the  course  or  even  simply  a 
slackening  of  speed,  and  the  aboral  turning  may  manifest 
itself  in  swinging  the  anterior  side  around  in  a  little  larger 
spiral  than  before.  In  fact  this  aboral  turm'ng  may  be  re- 
garded as  but  an  accentuation  of  the  process  which  in  ordi- 
nary swimming  keeps  the  aboral  side  facing  the  outside  of  the 
spiral.  Sometimes,  as  in  a  dilute  solution  of  sodium  chlor- 
ide, the  Paramcecium  stops  swimming  forward  and  turns 
around  aborally,  with  its  posterior  end  keeping  nearly  in 
one  spot  and  its  anterior  end  describing  a  wide  circle. 

Paramcecium,  as  we  have  described  in  a  previous  chapter, 
reacts  negatively  to  both  hot  and  cold  water. 

There  seems  to  be  no  orientation 
here  to  the  heat  rays;  when  Para- 
mcecium swims  to  a  colder  or  a 
warmer  region  it  gives  the  motor 
reflex  and  goes  in  another  direc- 
tion.    Ordinary    light    has    little 

effect  upon  Paramcecium  but  Her-    „ 

.      .  Fig.  10. — Course  of  Paramoe- 

t el  has  shown  that  it  gives  a  nega-    cium  in   a   drop   of  dilute 

.•  ,  chemical. 

tive    response    to    very    power- 
ful ultra-violet  rays.     Its  reactions  to  gravity  are  not  deci- 
ded and  are  influenced  by  various  factors  as  is  mentioned  in 
the  foregoing  section  on  Geotaxis. 

The  behavior  of  Paramcecium  is  made  up  of  a  very  limited 
number  of  stereotyped  modes  of  response.  It  reacts  to  all 
sorts  of  stimuli  by  the  same  motor  reflex  carried  out  in 
various  degrees  of  vigor  according  to  the  strength  of  the 
stimulus.  But  while  very  machine-like  its  behavior  is  at 
the  same  time  highly  plastic  and  adaptive.  Conditions 
which  are  unfavorable  act  as  stimuli,  and  the  animal  keeps 
on  reacting  until  it  gets  into  a  region  which  is  more  favorable 


76  THE  BEHAVIOR  OF  PROTOZOA 

to  its  life.  While  we  commonly  speak  of  the  positive  reac- 
tions of  the  organism,  strictly  speaking  there  are,  with  the 
exception  of  its  thigmotaxis,  no  positive  reactions.  If 
Paramoecia  collect  in  weak  acid  it  is  not  because  the  acid 
attracts  them  in  any  way;  they  are  not  stimulated  even 
when  they  accidentally  enter  the  acid;  they  react  only 
when  they  pass  from  the  acid  into  the  water.  It  is  not 
their  positive  reaction  to  the  acid  but  their  negative  reaction 
to  the  water  that  causes  what  appears  to  be  a  positive 
chemotaxis.  The  same  is  of  course  true  of  their  reactions 
to  various  other  stimuli.  Their  life  is  one  of  continual 
avoidance.  Only  when  conditions  are  favorable  is  there 
cessation  of  movement.  If  we  do  not  wish  to  attribute  to 
such  creatures  the  power  of  choice  we  must  admit  that  the 
method  of  behavior  secures  it  the  same  advantages  that 
choice  affords. 

The  behavior  of  Paramoecium,  like  that  of  every  other 
organism,  is  modified  by  changes  of  its  internal  condition. 
Strong  induction  shocks  render  Paramoecium  insensitive  to 
weaker  shocks  and  if  individuals  are  kept  for  a  time  at 
a  temperature  higher  than  normal  a  higher  temperature  is 
required  to  cause  the  avoiding  reaction.  Individuals  that 
have  been  kept  without  food  become  restless  while  well  fed 
ones  are  more  sluggish  and  more  apt  to  come  to  rest  against 
solid  objects.  Reactions  to  gravity  are  influenced  by  food 
and  other  conditions  and  thigmotaxis  is  markedly  affected  by 
temperature.  Behavior  may  be  modified  by  repetition  of  the 
same  contact  stimulus  as  described  in  the  following  quota- 
tion from  Jennings:  "If  a  bit  of  filter  paper  is  placed  in  a 
preparation  of  Paramoecia,  the  following  behavior  may  often 
be  observed.  An  individual  swims  against  it,  gives  the  avoid- 
ing reaction  in  a  slightly  marked  way,  swimming  backward  a 
little;  then  it  swims  forward  again,  jerks  back  a  shorter 


THE  BEHAVIOR  OF  PROTOZOA  77 

distance,  then  settles  against  the  paper  and  remains.  After 
remaining  a  few  seconds,  it  may  move  to  another  position, 
still  remaining  in  contact  with  the  paper.  Then  it  may 
leave  the  paper  and  go  on  its  way.  All  this  may  happen 
without  the  slightest  evident  change  in  the  outer  conditions. 
So  far  as  can  be  seen,  the  Paramoecium  first  responds  to  the 
solid  by  the  avoiding  reaction,  later  by  the  positive  contact 
reaction,  and  still  later  suspends  the  contact  reaction,  all  with- 
out any  change  in  external  conditions.  The  changes  inducing 
the  change  in  reaction  must  then  be  within  the  animal." 

The  behavior  of  Paramoecium  is  quite  typical  for  infusor- 
ians  in  general,  but  different  forms  present  some  interesting 
modifications.  Wliat  Maupas  has  called  the  ^  'hunter  ciliates" 
show  a  more  highly  developed  behavior  in  taking  food,  as 
they  not  only  exhibit  a  power  of  selection  of  certain  kinds  of 
food,  but  have  a  remarkable  power  of  engulfing  large  objects. 
One  of  those  whose  habits  are  the  best  known  is  Didinium 
nasutum  (Stein).  The  body  is  in  the  shape  of  a  barrel; 
at  the  middle  of  the  anterior  end  is  a  small  projection  where 
the  mouth  is  located.  The  mouth,  which  is  usually  kept 
closed,  leads  to  a  pharynx  lined  with  chitinous  rods  which 
can  act  as  a  sort  of  "seizing  organ,"  in  the  capture  of  prey. 
This  organ  can  be  protruded  and  withdrawn,  and  may  be 
opened  out  to  a  remarkable  degree. 

When  Didinium  in  the  course  of  its  swimming  comes  in 
contact  with  a  Paramoecium  or  other  small  organism,  the 
seizing  organ  is  shot  forth  into  the  body  of  the  prey.  The 
seizing  organ  is  then  drawn  in  and  the  mouth  spreads  open 
to  receive  the  prey  which  is  gradually  pulled  into  the  body 
of  the  Didinium. 

The  swallowing  capacity  of  Didinium  is  almost  incredible. 
Mast  observed  one  of  these  infusorians  which  swallowed  a 
Paramoecium  ten  times  as  large  as  its  captor,  the  latter 


78  THE  BEHAVIOR  OF  PROTOZOA 

appearing  as  if  forming  a  mere  film  over  its  engulfed  victim. 
Nothing  seems  too  large  for  Didinia  to  attack.  Their 
powers  of  digestion  seem  equal  to  their  voracity;  according 
to  Mast  a  Didinium  will  digest  an  ordinary  Paramoecium 
once  every  three  hours.  According  to  Balbiani  Didinium 
actively  pursues  its  prey  and  when  sufficiently  near  *'it 
begins  by  casting  at  it  a  quantity  of  bacillary  corpuscles 
which  constitute  its  pharyngeal  armature."  Coming  to 
closer  quarters  it  thrusts  forth  its  prehensible  apparatus 
into  its  victim  and  drags  it  back  toward  its  mouth.  This 
behavior  is  referred  to  by  Binet  as  a  ''most  complicated 
instance  of  localization '^  involving  a  precise  knowledge  of 
the  position  of  the  prey  at  which  the  Didinium  takes  a 
definite  aim. 

We  have  here  an  instance  of  how  easily  one  may  be  de- 
ceived in  interpreting  the  behavior  of  lower  organisms. 
There  is  no  evidence  that  Didinium  pursues  its  prey  like  a 
hunter.  Mast  has  shown  that  it  does  not  discharge  tri- 
chocysts  at  a  distance;  in  fact  it  has  none  to  discharge;  the 
loose  trichocysts  seen  when  Didinium  attacks  Paramoecium 
and  which  Balbiani  thought  were  shot  out  by  the  hunter  in- 
fusorian  were  derived  entirely  from  the  organism  attacked. 
According  to  Jennings,  Dininium  reacts  in  much  the  same 
way  "not  only  to  objects  which  may  serve  as  food,  but  to 
all  sorts  of  solid  bodies.  In  other  words,  the  process  is  one 
of  trial  of  all  sorts  of  conditions.  On  coming  in  contact 
with  a  solid,  Didinium  'tries'  to  pierce  and  swallow  it. 
If  this  succeeds,  well  and  good;  if  it  does  not,  something 
else  is  'tried.'  In  a  culture  containing  many  specimens  of 
Didinium,  the  author  has  seen  dozens  of  individuals  reacting 
in  this  way  to  the  bottom  and  sides  of  the  glass  vessel,  ap- 
parently making  persevering  efforts  to  pierce  the  glass. 
Others  'try'  water  plants,  or  masses  of  small  algae,  about 


THE  BEHAVIOR  OF  PROTOZOA  79 

which  many  specimens  gather  at  times.  Of  course  they  get 
no  food  in  this  way."  They  make  attempts  on  various  other 
organisms  with  which  they  come  into  contact,  but  often 
fail  on  account  of  the  toughness  or  lai-ge  size  of  the  organism 
attacked. 

What  seems  at  first  to  be  choice  of  prey  is  really  some- 
thing quite  different.  The  infusorian  swallows  what  it 
can  and  not  what  it  will.  ^'The  apparent  choice  of  food/' 
says  Mast,  "is  due  to  the  fact  that  the  seizing  organ  will 
adhere  to  some  organisms  and  not  to  others.  The  Didinia 
come  in  contact  with  all  sorts  of  objects  in  their  random 
swimming  and  attempt  to  swallow  all  those  to  which  the 
seizing  organ  will  adhere."  After  all  Didinium  like  Para- 
moecium  is  a  pretty  simple  sort  of  a  creature  in  its  be- 
havior. It  has  but  a  few  simple  tricks  which  it  tries 
over  and  over  again.  Its  going  gunning  with  its  armory 
of  projectiles,  its  accurate  sense  of  position  and  its  marks- 
manship are  all  creations  of  the  observer's  fancy. 

A  protozoan  exhibiting  somewhat  more  complex  behavior 
than  is  shown  by  Paramoecium  is  the  large  cihate  Loxophyl- 
lum  meUagris.  This  organism  usually  glides  along  the 
surface  of  objects  by  means  of  cilia  on  the  side  of  the  body, 
but  at  times  it  may  swim  in  a  spiral  manner  through  the 
water.  If  strongly  stimulated  while  swimming  it  may 
back  off  and  turn  in  another  direction  much  as  Paramoecium 
does,  but  the  organism  seems  to  be  averse  to  swimming  and 
generally  takes  advantage  of  the  first  opportunity  to  glide 
along  over  some  solid  object  or  the  surface  film.  These 
gliding  movements  are  carried  on  in  a  more  or  less  rythmical 
way  most  of  the  time.  The  body  is  narrowed  and  elongated 
and  swims  forward  for  a  short  distance,  then  there  is  a  con- 
traction resulting  in  making  the  body  shorter  and  broader 
and  at  the  same  time  a  reversal  of  the  effective  beat  of  the 


80  THE  BEHAVIOR  OF  PROTOZOA 

cilia  which  causes  a  backward  movement.  This  is  followed 
by  a  turning  to  one  side,  the  oral  in  this  case,  after  which 
Loxophyllum  swims  forward  in  a  different  direction.  The 
performance  is  repeated  time  after  time,  and  the  infusorian 
is  kept  circling  about  in  the  same  region,  for,  it  may  be, 
several  hours.  In  this  species  there  is  a  regular  association 
between  the  elongation  of  the  body  and  the  backward  strokes 
of  the  cilia,  and  between  the  contraction  of  the  body  and  the 
reversed  beat  of  the  cilia,  an  association  which  is  preserved 
even  in  the  movements  of  small  fragments  of  the  body. 
If  Loxophyllum  is  cut  into  several  pieces  these  pieces  may 
swim  in  a  spiral  course  or  glide  over  the  surface  of  objects 
much  like  the  entire  organism.  When  they  go  forward 
they  become  narrow  and  elongated  and  when  they  swim 
backward  they  become  shorter  and  wider,  and  they  perform 
these  movements  in  regular  alternation,  at  the  same  time 
circling  slowly  toward  the  oral  side.  The  factors  which 
determine  the  peculiar  traits  of  behavior  in  this  species  seem 
to  be  present  in  all  parts  of  the  body,  for  no  matter  how  minute 
the  fragments  into  which  the  organism  is  divided  the  action 
of  the  parts,  so  far  as  physical  conditions  permit,  is  the  same. 
Habitual  contact  with  solid  objects  seems  to  have  been  the 
cause  of  the  development  in  Loxophyllum  of  some  features  of 
behavior  not  found  in  other  swimming  infusoria.  There 
are  small  bendings  of  the  sensitive  anterior  end  in  various 
directions  as  if  it  were  feeling  its  way  along.  There  are 
undulations  of  the  margin  and  bendings  and  twistings 
of  the  whole  body.  The  food-taking  movements  as  described 
by  Oelzelt-Newin  are  quite  complex  and  involve  a  number 
of  coordinated  acts.  The  organism  glides  over  its  food 
and  when  the  large  slit-like  mouth  is  in  the  proper  position 
the  lips,  which  are  usually  tightly  closed,  open  and  begin  a 
series   of  spreading  movements  which  result  in  engulfing 


THE  BEHAVIOR  OF  PROTOZOA 


81 


the  object.  Then  there  are  righting  movements  which  are 
brought  into  play  when  Loxophyllum  is  turned  over  on 
its  left  side.  There  is  not,  as  we  might  be  led  to  expect, 
but  a  single  stereotyped  method  of  righting.  The  organism 
rights  itself  by  a  number  of  very  different  methods  which 
present  an  indefinite  number  of  modifications.  One  cannot 
but  wonder  when  watching  the  varied  movements  of  this 
graceful  and  supple  infusorian  that  a  single  cell  is  capable 
of  such  behavior. 

One  of  the  most  highly  developed 
types  of  behavior  which  has  been 
carefully  studied  in  the  protozoa  is 
exhibited  by  the  large  infusorian 
Stentor.  There  are  several  species 
of  this  genus,  but  all  are  trumpet- 
shaped,  with  a  mouth  situated  at 
the  bottom  of  a  depression  at  one 
side  of  the  anterior  end.  The  oral 
end  of  the  organism  is  surrounded 
by  a  zone  of  membranellse  which  at 
one  end  descends  in  a  spiral  course 
toward  the  mouth.  The  whole  sur- 
face of  the  body  is  covered  by  uni- 
form cilia,  with  the  exception  of  a 
small  area  of  naked  protoplasm  at 
the  small  end  or  foot,  by  means  of 
which  Stentor  is  able  to  attach  itself 
to  foreign  objects.  \Mien  free  in  the 
water  Stentor  is  able  to  swim  by 
the  action  of  its  cilia  and  membranellse.  Like  Paramoe- 
cium  it  follows  a  spiral  course  and  when  stimulated  it  may 
perform  the  motor  reflex,  backing  off  by  reversing  the 
beat  of  its  cilia,  turning  to  the  aboral  side  and  then  going 


Fig.  11.- 
morphus. 


-Stentor  poly- 
( After    Stein.) 


82  THE  BEHAVIOR  OF  PROTOZOA 

ahead  in  another  direction.  To  a  degree  unusual  among 
infusoria  Stentor  has  the  power  of  changing  the  form  of 
its  body.  It  may  extend  into  the  form  of  a  very  long  slender 
trumpet,  or  contract  almost  into  a  sphere.  The  ability 
to  undergo  these  changes  is  due  to  the  presence  of  numerous 
contractile  threads  or  myonemes  which  extend  for  the  most 
part  longitudinally  just  beneath  the  outer  layer  of  ecto- 
plasm. Food  taking  in  Stentor  is  accomplished  with  the  aid 
of  the  cilia  at  the  anterior  end  of  the  body  and  the  membran- 
ellae  leading  to  the  oral  opening.  The  currents  set  up  by 
the  beating  of  these  organs  carry  bodies  to  the  mouth 
which  has  the  power  of  taking  in  comparatively  large  ob- 
jects, for  one  often  sees  rotifers,  diatoms,  algae,  and  various 
protozoans  in  the  endoplasm  of  the  animal. 

While  swimming  freely  in  the  water  the  behavior  of 
Stentor  is  in  general  similar  to  that  of  Paramoecium.  The 
spiral  swimming  and  the  motor  reflex  in  response  to  chemical, 
mechanical,  thermal  and  electrical  stimuli  are  much  the 
same*  in  both  organisms.  Stentors  as  a  rule  react  to  light 
which  has  little  effect  on  Paramoecia.  In  Stentor  cceruleuSf 
which  is  negatively  phototactic,  sudden  illumination  evokes 
the  motor  reflex  which  after  one  or  more  trials  enables  the 
animal  to  reach  a  more  shaded  region.  The  anterior  end  is 
the  region  most  sensitive  to  light  and  when  the  organism 
is  pointed  toward  the  light  it  gives  the  motor  reflex,  and 
swims  in  a  different  direction.  If  still  pointing  obliquely 
toward  the  light  it  may  repeat  the  motor  reflex  and  continue 
to  do  so  until  its  anterior  end  is  directed  away  from  the 
source  of  stimulation,  when  the  Stentor  swims  off  in  the 
direction  of  the  rays. 

When  Stentor  is  attached  it  exhibits  several  peculiar  types 
of  activity.  It  may  contract  or  extend  the  body,  and  it 
often  sways  about  in  various  directions  in  a  more  or  less 


THE  BEHAVIOR  OF  PROTOZOA  83 

rhythmical  manner.  To  contact  stimuli  Stentor  responds 
in  a  variety  of  ways  dependent  on  the  strength  of  the 
stimuli  and  the  number  of  times  they  have  been  repeated. 
A  moderately  strong  stimulus  causes  the  Stentor  to  contract 
violently,  but  after  a  number  of  repetitions  the  contractions 
diminish  in  vigor  and  finally  disappear.  Jennings  found 
that  when  Stentor  is  stimulated  by  a  quantity  of  fine  par- 
ticles of  India  ink  or  carmine  which  were  poured  upon  the 
disk  by  a  capillary  pipette,  a  regular  series  of  responses 
was  given.  Frequently  the  Stentor  would  not  respond  at 
first,  but  would  sweep  the  particles  into  its  gullet  con- 
tinuously. Sooner  or  later,  however,  the  organism  would 
respond  by  bending  to  the  aboral  side.  This  may  be 
repeated  several  times,  but  if  it  fails  to  afford  relief  from 
the  stimulation  another  reaction  is  "tried."  There  is  a 
sudden  reversal  of  the  action  of  the  cilia  and  the  par- 
ticles are  then  thrown  off  the  disk.  The  response  is  but 
momentary,  however,  and  then  the  usual  movements  are 
resumed.  If  these  two  reactions  are  fruitless  the  Stentor 
contracts  strongly,  thus  drawing  its  body  out  of  the  re- 
gion of  the  impending  particles.  After  a  little  it  slowly 
extends  again,  and  if  the  particles  still  fall  on  the  disk  the 
contraction  may  be  repeated.  If  the  stimuli  still  come 
after  a  number  of  such  attempts  to  avoid  them  the  Sten- 
tor makes  several  violent  contractions  in  quick  succession 
and  breaks  loose  from  its  attachment  and  swims  away. 
We  have  a  series  of  reactions  to  the  same  external  stimulus. 
If  one  reaction  is  unsuccessful  another  is  tried  until  the 
organism  finally  obtains  relief.  These  reactions  are  all  of  an 
adaptive  character,  so  we  can  say  that  the  creature  is  pro- 
vided with  a  number  of  ways  of  meeting  a  given  situation. 
The  external  stimulus  remaining  the  same,  the  particular 
reaction  that  is  given  obviously  depends  upon  the  condition 


84  THE  BEHAVIOR  OF  PROTOZOA 

of  the  organism.  For  the  physiological  state  A  there  is  one 
reaction,  for  state  B  another,  and  so  on.  As  in  a  higher 
animal  the  behavior  of  Stentor  depends  upon  its  previous 
history.  In  a  sense  the  organism  may  be  said  to  profit  by 
experience,  although  it  cannot  be  said  to  learn,  because 
there  is  no  formation  of  new  associations  such  as  occurs 
in  the  learning  of  higher  forms. 

What  are  these  internal  physiological  states  to  which  the 
adaptive  changes  of  behavior  in  Stentor  are  due?  We 
have  here  two  kinds  of  modification,  as  Jennings  has  pointed 
out,  the  failure  to  respond  to  a  stimulus  which  at  first  evoked 
a  reaction,  and  the  replacement  of  one  reaction  by  another. 
In  regard  to  the  first  modification  Jennings  remarks  that 
"It  seems  improbable  that  the  change  of  behavior  is  due 
to  fatigue,  since  the  change  occurs  after  but  a  single  stimula- 
tion and  a  single  contraction.  It  could  hardly  be  supposed 
that  these  would  fatigue  the  animal  to  such  an  extent  as  to 
prevent  further  contraction.  And  if  we  use  stronger 
stimuli,  we  find  that  the  animal  continues  to  contract  suc- 
cessively every  time  the  stimulus  is  applied,  for  an  hour  or 
more."  Even  after  the  animal  has  ceased  to  contract 
strongly  it  may  respond  to  a  stimulus  by  bending  to  one  side, 
thus  showing,  according  to  Jennings,  that  the  failure  to 
contract  is  not  due  to  "a  fatigue  of  the  perceptive  power, 
for  the  bending  into  a  new  position  shows  that  the  stimulus 
is  perceived,  though  the  reaction  differs  from  the  first  one.'* 
Fatigue  is  usually  associated  in  our  minds  with  a  condition 
indicative  of  exhaustion,  and  of  such  Stentor  certainly  gives 
no  evidence  after  a  few  stimulations,  but  that  there  may  be 
a  slight  degree  of  essentially  the  same  state  which  when  carried 
further  we  designate  as  fatigue,  seems  to  me  a  possibility 
not  ruled  out  by  the  experiments.  It  is  possible,  however, 
that  the  change  in  question  may  be  due  to  something  simi- 


THE  BEHAVIOR  OF  PROTOZOA  85 

lar  to  the  phenomenon  of  acclimatization  to  chemicals  and 
other  agencies  which  is  apparently  a  fundamental  charac- 
teristic of  living  organisms. 

The  replacement  of  reactions  by  Stentor  probably  con- 
sists to  a  considerable  degree  in  variations  in  the  vigor  and 
completeness  of  a  single  reaction.  A  gentle  contraction  may, 
owing  to  the  bodily  peculiarities  of  the  animal,  involve  a 
turning  to  the  aboral  side,  as  its  easiest  channel  of  expres- 
sion. With  stronger  contraction  this  would  naturally  be 
obscured,  so  that  an  apparently  new  response  may  result 
from  the  heightened  irritability  caused  by  the  preceding 
stimuli.  The  succession  of  contractions  resulting  in  the 
separation  of  the  organism  from  its  attachment  naturally 
falls  under  the  same  interpretation,  and  is  analagous  to 
what  takes  place  in  an  excised  heart  which,  upon  being 
given  a  single  stimulus,  may  contract  once  or  several  times 
according  to  its  condition  of  irritability.  The  successive 
contractile  phenomena  in  Stentor  are  more  or  less  analagous  to 
those  of  summation  of  stimuli  in  an  excised  muscle.  The  re- 
versal of  the  beat  of  the  cilia  is  a  separate  reaction,  although 
it  may  have  a  certain  relation  to  the  phenomenon  of  con- 
traction. We  are  not  justified  in  assuming  that  Stentor 
passes  through  a  number  of  discrete  internal  states  each 
of  which  has  a  correspondingly  discrete  motor  response. 
The  number  of  physiological  states  as  in  every  organism 
is  unlimited  and  the  behavior  of  the  animal  shows  us  a 
series  of  reactions  differing  for  the  most  part  in  degree  of 
vigor  rather  than  in  kind,  like  the  motor  reflex  in  Paramoe- 
cium,  which  may  be  carried  out  in  various  degrees  of  com- 
pleteness from  a  momentary  slowing  of  speed  to  a  prolonged 
backward  swimming  followed  by  numerous  rotations  toward 
the  aboral  side. 

Schaeffer  has  carried  on  a  series  of  careful  experiments  in 


86  THE  BEHAVIOR  OF  PROTOZOA 

which  it  was  shown  that  when  Stentor  was  offered  either 
alternately  or  at  the  same  time  nutritious  objects  such  as 
Phacus  or  Euglena  and  such  substances  as  starch  grains, 
powdered  carmine  or  India  ink,  fine  sand  or  sulphur,  the 
former  would  be  swept  into  the  gullet  and  ingested,  while 
the  latter  would  usually  be  rejected.  Conditions  of  hunger 
or  satiety  influence  the  selection  of  food.  Very  hungry 
Stentors  may  ingest  indigestible  particles  of  carmine  or 
India  ink,  but  when  better  fed  the  discrimination  is  more 
precise  and  only  digestible  material  is  taken  in.  Hungry 
Stentors  differ  from  well  fed  ones  also  in  the  greater  extension 
of  the  body  and  the  greater  activity  of  the  membranellae, 
but  they  are  less  responsive  to  mechanical  stimuli. 

The  behavior  of  Protozoa,  as  we  have  seen,  is  influenced 
by  their  previous  activity  as  well  as  by  changes  of  external 
conditions.  That  behavior  should  be  modified  by  these 
things  is  of  course  inevitable,  for  no  organism  is  ever  twice 
the  same,  and  the  life  of  every  organism  is  one  of  constant 
adjustment  to  the  external  world.  How  far  these  changes 
indicate  the  presence  of  mind  is  a  question  about  which 
there  is  much  dispute.  These  changes  are  to  a  considerable 
degree  of  an  adaptive  nature,  but  the  same  may  be  said  of 
many  purely  physiological  processes  occurring  in  our  bodies. 
There  are  some  phenomena  described  which  have  been 
interpreted  as  the  acquirement  of  habit  and  even  as  learning 
by  experience,  but  the  observations  on  this  score  scarcely 
justify,  in  the  opinion  of  the  writer,  the  interpretations 
that  have  been  placed  upon  them.  Mr.  Stevenson  Smith 
has  performed  some  experiments  which  lead  him  to  the 
conclusion  that  Paramoecium  is  able  to  acquire  advantageous 
habits.  He  placed  a  Paramoecium  in  a  fine  tube  containing 
a  small  amount  of  water.  The  inner  diameter  of  the  tube 
was  less  than  the  length  of  the  Paramoecium,  so  that  the 


THE  BEHAVIOR  OF  PROTOZOA  87 

creature  had  great  difficulty  in  reversing  its  course,  having 
to  bend  its  body  in  the  form  of  a  U  to  get  around.  If  the 
diameter  of  the  tube  is  not  too  small  the  time  which  it  takes 
the  Paramcecium  to  turn,  says  Smith,  "may  gradually  be 
shortened  and  a  most  surprising  aptitude  of  turning  be 
developed.  ...  I  have  found  a  reduction  of  turning 
time,  after  the  animals  have  been  in  the  tube  for  twelve 
hours  or  more,  from  four  to  five  minutes  to  a  second  or  two, 
which  is  the  minimum  time  in  which  the  turns  can  be 
made."  Although  Paramoecia  kept  without  food  for  twelve 
hours  would  diminish  sufficiently  in  size  to  enable  them  to 
turn  within  the  tube  with  much  greater  ease — a  fact  which 
Mr.  Smith  apparently  has  not  considered — Day  and  Bentley, 
who  have  repeated  Smith's  experiments,  have  found  that  the 
greater  faciHty  in  turning  is  acquired  within  a  few  minutes. 
It  should  be  borne  in  mind  that  Paramcecium  is  an  organ- 
ism which  takes  in  and  excretes  water  many  times  more 
rapidly  than  even  the  specialized  organs  of  excretion  of 
higher  animals,  and  that  the  abnormal  conditions  resulting 
from  confinement  within  a  very  small  amount  of  water 
may  possibly  cause  a  certain  change  in  size  within  a  short 
time.  I  have  often  observed  a  marked  shrinkage  in  Para- 
moecia when  they  are  placed  in  a  medium  of  somewhat 
higher  osmotic  pressure.  In  abnormal  conditions  Paramoecia 
become  more  plump  and  the  body  seems  softer  and  more 
flexible,  and  it  is  also  possible  that,  since  Paramcecium  is 
endowed  with  a  certain  degree  of  contractility,  the  stimuli 
encountered  through  its  frequent  efforts  to  turn  within  the 
tube  might  cause  a  shortening  of  the  body  which  would  cer- 
tainly occur  in  a  more  marked  way  in  a  Stentor  and  many 
other  infusorians  under  these  conditions.  The  experiment 
has  its  practical  drawbacks  as  a  means  of  testing  habit  for- 
mation or  "learning,"  since  the  changes  of  size,  form  and  con- 


88  THE  BEHAVIOR  OF  PROTOZOA 

sistency  which  the  animals  might  undergo  would  so  greatly 
influence  the  result.  The  fact  described  by  Day  and  Bentley 
that  Paramoecia  which  had  acquired  a  facility  of  turning 
still  showed  the  effects  of  their  experience  after  having  been 
placed  for  an  interval  of  twenty  minutes  in  their  culture 
medium  may  well  be  due  to  the  persistence  of  the  purely 
physiological  or  pathological  effects  of  their  previous  con- 
finement. 

Smith  devised  another  experiment  to  show  the  modifica- 
tion of  the  reaction  of  Paramoecium  to  changes  of  temperature. 
A  tube  containing  several  Paramoecia  was  so  arranged  that 
either  end  could  be  heated  or  cooled  at  will.  A\Tien  one  end 
was  heated  the  Paramoecia  would  dart  about  at  random,  and 
when  they  swam  into  the  cooler  water  they  would  often  turn 
back  to  the  hot  water  again.  After  the  temperature  of  the 
two  ends  of  the  tube  was  reversed  the  animals  would  dart 
about  much  as  before,  reaching  the  cool  water  only  after  a 
number  of  trials.  With  repeated  reversals,  however,  the 
movements  of  the  Paramoecia  became  "slower  and  more 
regulated"  and  they  would  "seldom  turn  more  than  once 
toward  the  cold  water  before  swimming  in  that  direction.^' 
According  to  Smith,  Paramoecium  does  not  give  evidence  of 
the  possession  of  associative  memory,  but  he  concludes  that 
"its  behavior  may  be  modified  to  show  the  results  of  practice, 
both  in  a  reduction  of  the  time  involved  in  performing  a 
movement  and  in  the  increase  of  the  suitability  of  the  move- 
ment to  accomplish  the  appropriate  result."  The  modified 
reaction  to  temperature,  I  believe,  may  be  accounted  for  not 
so  much  through  the  effect  of  practice  in  the  performance 
of  an  act,  but  as  a  consequence  of  a  general  diminution  of 
excitability.  Take  a  few  drops  of  a  Paramoecium  culture 
and  place  them  on  a  slide.  For  a  time  the  animals  scurry 
about  in  the  greatest  haste  and  confusion,  and  frequently 


THE  BEHAVIOR  OF  PROTOZOA       89 

give  the  motor  reflex  with  no  apparent  cause.  After  a  time 
tkeir  actions  become  slower  and  more  sober.  Have  we  any- 
thing essentially  different  in  the  experiments  of  Mr.  Smith? 
Possibly  so,  but  I  cannot  convince  myself  of  it  from  the 
results  described. 

Hodge  and  Aikins  in  their  study  of  the  daily  life  of  a 
Vorticella  observed  that  one  individual,  after  having  en- 
gulfed yeast  cells  for  some  time,  refused  them  and  persisted 
in  so  doing  for  several  hours.  ^^Hiat  this  fact  signifies  can- 
not be  decided  from  the  single  observation  reported;  there 
are  a  number  of  possibilities,  and  the  correct  interpretation 
can  be  made  only  after  carefully  planned  experiments. 

There  have  been  few  systematic  investigations  with  the 
end  of  testing  the  educability  of  the  protozoa,  and  while 
granting  the  possibility  that  future  work  may  compel  U3 
to  modify  our  conclusion,  it  may  be  said  that,  thus  far,  there 
is  no  unmistakable  evidence  that  the  protozoa  are  capable 
of  forming  true  habits  or  of  learning  by  association. 

BIBLIOGRAPHY 

BiNET,  A.  The  Psychic  Life  of  Micro-organisms.  Chicago,  '94. 
Day,  L.  M.  and  Bentley,  M.    A  Note  on  Learning  in  Paramoecium. 

Jour.  An.  Behavior,  1,  67,  '11. 
Dellinger,  0.  P.     Locomotion  of  Amoebae  and  allied  forms.    Jour. 

Exp.  Zool.,  3,  337,  '06. 
GiBBS,  D.  and  Bellinger,  0.  P.     The  Daily  Life  of  Amoeba  proteus. 

Am.  Jour.  Psych.,  19,  232,  '08. 
Hodge,  C.  F.  and  Aikins,  H.  A.    The  Daily  Life  of  a  Protozoan. 

Am.  Jour.  Psych.,     6,  524,  '95. 
Holmes.   S.  J.     The   Behavior  of  Loxophyllum,  etc.  Jour.   Exp. 

ZooL,  4,  306,  '07.     Rhythmical  Activity  in  Infusoria.     Biol. 

Bull.,  13,  306,  '07. 
Jennings,  H.  S.     Contributions  to  the  Study  of  the  Behavior  of 

Lower   Organisms.     Carnegie   Inst.    Pubs.    Wash.,    '04.     The 

Behavior  of  Paramoecium.     Additional  Features  and  General 

Relations.     Jour.  Comp.  Neur.  Psych.  14,  441,  '04.     Behavior 

of  Lower  Organisms.     N.  Y.,  '06. 


90  THE  BEHAVIOR  OF  PROTOZOA 

Mast,  S.  0.     The  Reactions  of  Didinium  nasutum   (Stein)  with 

Special  Reference  to  the  Feeding  Habits  and  the  Functions  of  the 

Trichocysts.     Biol.  Bull.,  16,  91,  '09. 
Oelzelt-Newin,  a.     Beobachtungen  liber  das  Leben  der  Proto2oen 

Zeit.  f.  Psych,  und  Physiol,  der  Sinnesorgane,     41,  349,  '06. 
Prowazek,  S.  von.     Einfiihrung  in  die  Physiologie  der  Einzelligen 

(Protozoen).     Leipzig  and  Berlin,  '10. 
PtJTTER,  A.  Die  Reizbeantwortungen  der  ciliaten  Infusorien.     Zeit. 

f.  allg.  Physiol.,     3,  406,  '00. 
Rhumbler,  L.  Physikalische  Analyse  von  Lebenserscheinungen  der 

Zelle.  I,  Bewegung,  Nahrungsaufnahme,  Defakation,  Vacuolen- 

Pulsation  und   Gehausebau  bei  lobosen  Rhizopoden.     Arch.  f. 

Entwicklungsmech.,  7,  103,  '98.     Zur  Theorie  der  Oberflachen- 

krafte  der  Amoben.     Zeit.  f.  wiss.  Zool.,     83,  1,  '05.     Die  ver- 

schiedenartigen    Nahrungsaufnahmen   bei   Amoben   als   Folge 

verschiedener    Colloidzustande    ihrer    Oberflachen.     Arch.    f. 

Entwicklungsmech.  30,  194,  '10. 
ScHAEFFER,   A.    A.     Selection   of   Food   in   Stentor.     Jour.    Exp. 

Zool.,  8,  75,  '10. 
Smith,    9.     The    Limits   of    Educability   in   Paramoecium.     Jour. 

Comp.  Neur.  Psych.,  18,  499,  '08. 
Verworn,    M.    Psycho-physiologische    Protistenstudien.     Experi- 

mentelle  Untersuchungen,  Jena,  '89. 
Watkins,  G.  p.    Psychical  Life  in  Protozoa.     Am.  Jour.  Psych., 

11,  166,  '00. 


CHAPTER  V 
INSTINCT 

"L'instinct  sait  tout,  dans  les  voies  invariables  qui  lui  ont  6iA 
trac^es;  il  ignore  tout,  en  dehors  de  les  voies.  Inspirations  sublimes 
de  science,  inconsequences  ^tonnantes  de  stupidity,  sont  k  la  fois  son 
partage,  suivant  que  I'animal  agit  dans  des  conditions  normales  ou 
dans  des  conditions  accidentelles." — ^Fabre.  Souvenirs  EntomO' 
logigues.  T.  I. 

"We  are  governed  by  instinct,  as  well  as  cats  and  goats." — 
Voltaire.     Philosophical  Dictionary. 

While  it  may  not  be  necessary  to  define  a  term  so  well  known 
as  instinct,  it  may  not  be  without  interest  to  quote  the  follow- 
ing definitions  which  have  been  given  by  various  writers: 

"  We  may  call  the  instincts  of  animals  those  faculties  implanted  in 
them  by  the  Creator,  by  which,  independent  of  instruction,  observa- 
tion or  experience,  and  without  a  knowledge  of  the  end  in  view,  they 
are  all  ahke  impelled  to  the  performance  of  certain  actions  tending  to 
the  well  being  of  the  individual  and  the  preservation  of  the  species." — 
KiRBY  AND  Spence,  Introduction  to  Entomology,  1858. 

"A  propensity  prior  to  experience  and  independent  of  instruction." 
— ^Paley,  Natural  Theology. 

"An  action,  which  we  ourselves  require  experience  to  enable  us  to 
perform,  when  performed  by  an  animal,  more  especially  a  very 
young  one,  without  experience,  and  when  performed  by  many 
individuals  in  the  same  way,  without  their  knowing  for  what 
purpose  it  is  performed,  is  usually  said  to  be  instinctive.  But  I 
could  show  that  none  of  these  characters  are  universal." — Darwin, 
Origin  of  Species. 

Instinct  is  "compound  reflex  action." — Herbert  Spencer, 
Principles  of  Psychology. 

"  Instinct  is  a  general  term  comprising  all  those  faculties  of  mind 
which  lead  to  the  performance  of  actions  that  are  adaptive  in  char- 
acter, but  pursued  without  necessary  knowledge  of  the  relation  be- 

91 


92  INSTINCT 

tween  the  means  employed  and  the  end  attained." — Romanes, 
Article  Instinct,  Encyclopedia  Britannica. 

Instinct — "Purposeful  action  without  consciousness  of  the  pur- 
pose."— Von  Hartmann,  The  Philosophy  of  the  Unconscious. 

''Instinct  is  inherited  faculty,  especially  is  inherited  habit." — 
EiMER,  Organic  Evolution. 

"  Qu'est-ce  que  I'instinct?     Un  mot." — G.  Bohn. 

"L'Instinct  n'est  rien." — Condillac. 

The  above  definitions  show  how  differently  instinct  has 
been  conceived  as  regards  its  causation,  although  as  to  the 
kind  of  behavior  to  which  the  term  is  applied  there  is  in 
general  a  broad  basis  of  agreement.  Some  modern  writers 
would  have  us  discard  the  term  instinct  entirely  on  account 
of  its  vagueness  and  because,  as  commonly  used,  it  carries 
with  it  certain  connotations  of  which  they  do  not  approve. 
''Instinct,"  says  Bohn,  "is  a  legacy  of  the  past,  the  middle 
ages,  the  theologians  and  the  metaphysicians" — a  word 
which  does  not  stand  for  any  well-defined  reality.  Consign 
it  therefore  to  the  dust  bin,  and  describe  behavior  in  other 
and  more  scientific  terms.  Instinct  is  a  word  whose  con- 
notation very  naturally  has  varied  according  to  the  scientific 
and  philosophical  views  of  the  writers  who  have  employed  it, 
but  if  we  were  to  reject  terms  generally  on  this  ground  our 
language,  even  in  science,  would  undergo  an  embarrassing 
amount  of  modification.  There  are  few  scientific  terms, 
especially  in  psychology,  which  we  should  be  willing  to  accept 
to-day  with  the  meanings  they  had  a  hundred  years  ago. 
Stripped  of  its  older  metaphysical  implications,  which 
need  not  annoy  us,  and  used  to  designate  certain  types  of 
behavior,  the  term  is  a  very  useful  one.  It  may  not  be  pos- 
sible to  define  it  with  precision.  It  is  also  difficult  to  define 
a  child,  a  shrub  or  a  tree.  We  know  with  a  fair  degree  of 
clearness  what  is  meant  by  the  statement  that  nest  building 
in  birds  and  comb  making  in  bees  are  instinctive.     The 


INSTINCT  93 

same  idea  may  be  expressed  by  the  use  of  "brand-new" 
scientific  terms  which  have  never  been  soiled  by  theologians 
and  metaphysicians,  but  the  need  for  a  general  term  for 
kinds  of  behavior  commonly  classed  as  instinctive  would 
still  remain,  and  despite  the  efforts  of  a  few  comparative 
psychologists,  the  word  instinct  will  I  think  continue  in 
reputable  use. 

Illustrations  of  instinct  abound  everywhere  and  a  very 
few  will  suffice.  A  flesh  fly  when  first  emerging  from  its 
pupa  case  is  very  soon  ready  for  performing  the  various 
functions  of  its  life.  It  guides  itself  accurately  in  flight, 
and  deftly  escapes  its  would  be  captors  by  quick  and  appro- 
priate movements.  It  is  drawn  by  the  sense  of  smell  to 
suitable  objects  for  food.  It  avoids  various  kinds  of  injur- 
ious stimuli.  It  recognizes  out  of  a  vast  number  of  animate 
objects  the  opposite  sex  of  its  own  species.  When  ready 
to  deposit  its  eggs  it  selects,  out  of  a  great  variety  of  materials 
the  proper  substances  to  afford  food  for  its  future  larvae. 
Its  acts  are  unguided  by  previous  experiences;  they  are  not 
prompted  by  reflection  or  thought;  they  result  from  a  blind 
impulse  urging  the  insect  to  discharge  its  energies  in  certain 
specific  ways  without  knowing  why.  An  organism  of  the 
degree  of  psychic  development  of  a  flesh  fly  may  modify  its 
acts  to  a  certain  degree  through  the  effects  of  experience, 
but  as  a  matter  of  fact  such  modification  plays  but  a  small 
part  in  the  creature's  life. 

Some  years  ago  the  writer  studied  the  behavior  of  a  species 
of  amphipod,  Amphithoe  longiniaiia,  and  compared  the 
activities  of  the  adult  with  those  of  the  newly  hatched  young. 
Amphithoe  lives  in  tubular  nests  which  are  usually  lodged 
among  sea  weed.  The  nests  are  somewhat  longer  than  the 
animal  and  are  spun  of  a  w^b-like  material  into  which  bits 
of  sea  weed  are  often  incorporated  which  help  to  conceal 


94  INSTINCT 

the  occupant.  In  its  nest  Amphithoe  lies  in  wait  for  prey, 
ready  to  dart  out  upon  any  small  creature  which  touches 
the  ends  of  its  long  antennae. 

The  activities  of  the  adult  Amphithoe,  with  the  exception  of 
those  concerned  in  reproduction,  are  almost  exactly  parallelled 
by  those  of  its  young.  I  have  taken  the  eggs  from  the  mater- 
nal brood  pouch  shortly  before  hatching  and  kept  them 
isolated  in  individual  dishes.  For  some  time  after  emerging 
from  the  egg  the  young  were  weak  and  had  imperfect  control 
of  their  movements,  which  were  jerky  and  irregular.  Soon  the 
minute  creatures  could  crawl  and  swim  much  like  the  adults, 
and  the  next  day  they  began  constructing  nests  which  were 
the  same  in  shape  as  those  formed  by  their  parents.  The 
attitudes  in  the  nest,  the  waving  of  the  antennae,  the  beating 
of  the  swimmerets,  the  restless  movements  of  the  legs  and 
mouth  parts,  springing  after  food,  belligerency  toward 
passers  by,  the  little  unobtrusive  signs  of  timidity,  the 
reversal  of  position  in  the  nest  on  the  approach  of  danger 
and  the  general  behavior  outside  of  the  nest  were,  on  the 
next  day  after  hatching,  almost  exactly  the  same  as  in  older 
individuals.  The  only  differences  in  behavior  were  due  to 
the  feebleness  of  the  young  and  their  imperfect  control  of 
their  movements.  The  young  are  hatched  with  all  the 
instincts  necessary  fully  to  equip  them  for  the  business  of 
life.  No  experience  is  necessary  to  teach  them  what  is  ad- 
vantageous for  them  to  do. 

It  is  this  skill  and  apparent  foresight  exhibited  in  instinc- 
tive behavior  that  gives  rise  to  a  popular  notion  that  animals 
are  somehow  mysteriously  endowed  with  a  knowledge  of 
those  things  which  are  necessary  for  their  life.  It  is  some- 
times asserted  that  young  ducklings  will  make  for  the  nearest 
water  before  they  have  gained  any  experience  of  the  neighbor- 
hood and  in  the  absence  of  any  signs  by  which  the  presence 


INSTINCT  95 

of  water  might  be  indicated;  the  ducklings  just  know 
instinctively  where  water  is  to  be  found.  A  friend  of  mine 
once  urged  as  a  fact  of  which  our  boasted  scientific  theories 
of  instinct  give  no  explanation  the  circumstance  that  the 
burrowing  mammals  of  California,  before  an  unusually 
rainy  season,  would  leave  their  holes  near  the  gulches  and 
migrate  to  the  hillsides.  Long  before  any  indications  were 
furnished  to  the  weather  bureau  the  instinct  of  these  animals 
was  said  to  warn  them  of  the  danger  of  floods.  Some  time 
previous  to  the  conversation  the  mammals  were  said  to 
have  emigrated  from  the  lower  parts  of  the  vaUeys,  and 
therefore  a  rainy  winter  was  predicted.  It  so  turned  out, 
however,  that  the  season  in  question  proved  to  be  an  unusu- 
ally dry  one,  and  the  wonderful  instinct  of  the  burrowing 
mammals  gave  them  a  false  alarm.  The  migration  of  the 
mammals  may  or  may  not  have  taken  place  as  reported, 
but  the  episode  illustrates  a  very  prevalent  misconception  of 
the  nature  and  possibihties  of  instinct.  Instincts  frequently 
have  a  relation  to  future  events,  but  that  they  involve  a 
mysterious  knowledge  of  things  unpredictable  by  human 
reason  still  awaits  proof. 

With  all  their  wonderful  adaptiveness  instincts  are  far 
from  ideally  perfect  Much  of  Mark  Twain's  remarks  on 
the  futility  and  imbecility,  the  wasted  effort  and  labor  at 
cross  purposes  shown  in  the  behavior  of  ants  may  easily  be 
verified  by  any  observer.  Flesh  flies  will  deposit  their 
eggs  on  the  carrion  plant  {Stapelia  hirsuta)  whose  odor 
resembles  that  of  decaying  meat  on  which  the  eggs  are 
usually  laid.  The  domestic  hen  will  sometimes  attempt  to 
hatch  out  corn  cobs  or  other  inanimate  objects,  and  her 
maternal  instincts  will  lead  her  to  foster  ducklings  as  readily 
as  her  own  kin.  Sometimes  animals  devour  their  own 
eggs  or  young,  as  I  have  several  times  observed  in  cray- 


96  INSTINCT 

fishes  and  spiders;  and  in  some  centipedes  the  males  will  uni- 
formly devour  the  eggs  if  they  are  not  concealed  by  the 
female.  Darwin  states  that  in  a  South  American  species  of 
Molothrus  the  instincts  for  securing  proper  care  of  the  eggs 
are  so  imperfect  that  numerous  eggs  are  simply  dropped  on 
the  ground  and  abandoned. 

Ma^  instincts  are  at  first  not  clearly  defined.  The 
young  chick  pecks  at  all  sorts  of  small  objects  of  good  and 
bad  taste  alike.  The  young  lamb  will  follow  any  sort  of 
moving  object  of  a  certain  size  as  well  as  its  own  mother. 
It  is  said  to  suck  indefi-nitely  at  a  piece  of  wool  unless  guided 
by  some  fortunate  circumstance  to  the  proper  fount  of 
nutriment.  Young  terns  and  many  other  young  birds  will  not 
at  first  distinguish  their  parents,  but  will  cuddle  under  one's 
hand  in  perfect  confidence  and  contentment.  They  react 
in  much  the  same  way  to  a  great  variety  of  large  moving 
objects.  In  most  cases  these  mean  the  parent  birds,  and 
the  instinct  of  the  young  becomes  directed  to  their  parents 
because  the  latter  were  the  first  living  objects  coming 
within  their  experience.  Foster  mothers  of  various  kinds 
are  adopted  by  many  .young  birds  and  mammals  as  readily 
as  members  of  their  own  species. 

While  many  instincts  exist  in  a  completely  developed  state 
when  the  animal  first  enters  upon  active  life,  others  are 
manifested  only  when  it  has  reached  a  certain  degree  of 
maturity.  Such  have  been  called  by  Lloyd  Morgan  "de- 
ferred^ instincts."  Instinctive  fear  in  birds  may  not  appear 
at  first,  but  only  after  several  days.  Young  nestling  terns 
which  for  a  short  time  after  hatching  will  cuddle  contentedly 
under  one's  hands,  behave  very  differently  before  they  are  a 
third  grown.  They  then  scuttle  away  in  wildest  alarm 
upon  one's  approach  and  hide  by  crouching  down  in  the  grass, 
where  they  will  lie  perfectly  quiet.     The  instinct  of  feigning 


INSTINCT  97 

death  which  does  not  occur  in  the  young  fledgling  now  ap- 
pears on  the  scene;  the  young  birds  will  allow  themselves  to 
be  handled  and  pulled  about  without  betraying  a  sign  of  hfe, 
and  will  even  suffer  their  tail  or  wing  feathers  to  be  pulled 
out  one  by  one  without  a  wince.  After  a  time,  as  if  the 
bird  recognized  the  futility  of  the  ruse,  the  death  feint  is 
discontinued  with  a  surprising  suddenness  to  be  followed 
by  violent  struggles,  screams  and  pecking  at  its  captor  in 
its  effort  to  make  its  escape.  Later  when  the  birds  are  able 
to  fly  the  crouching  and  death  feigning  instincts  disappear. 

With  birds  which  are  hatched  in  a  helpless  and  almost  feath- 
erless  condition  most  of  the  instincts  of  the  species,  with  the 
exception  of  opening  the  mouth  for  food  upon  the  appearance 
of  a  large  moving  object  in  their  vicinity,  are  in  abeyance. 
In  such  forms  running,  pecking,  flight,  etc.,  are  none  the 
less  instinctive;  they  are  simply  kept  from  appearing  on 
account  of  immaturity.  The  young  of  the  mound-building 
bird  will  take  wing  when  first  hatched  from  the  egg,  but  young 
swallows  are  not  able  to  fly  imtil  after  some  weeks.  That 
previous  experience  is  not  necessary  to  enable  them  to  ac- 
complish this  feat  is  shown  by  the  fact  that  young  nestlings 
kept  where  they  had  no  opportunity  to  use  their  wings 
until  they  were  of  the  proper  age  for  flight,  were  able 
to  fly  at  their  first  attempt  with  perfect  ease.  That  young 
birds  are  taught  to  fly  by  their  parents  is  a  popular  myth. 
The  instinct  to  fly  is  there  in  every  case;  its  appearance  is 
merely  deferred,  like  the  mating  and  nest  building  instincts, 
until  the  bird  reaches  a  certain  degree  of  development. 
Similarly  with  the  running  and  swimming  of  mammals. 
A  young  puppy  placed  in  the  water  will  flounder  helplessly 
and  soon  drown,  but  if  an  older  dog  is  thrown  into  the  water, 
though  he  may  never  have  been  in  the  water  before,  he  will 
swim  toward  the  shore. 

7 


98  INSTINCT 

With  animals  which  go  through  a  profound  metamorphosis 
in  the  course  of  their  development  we  find  correspondingly 
great  changes  in  instinctive  behavior.  Nothing  could  be, 
more  dissimilar  than  the  instincts  of  the  stealthy  dragon- 
fly nymph  which  prowls  among  the  debris  at  the  bottom  of 
-  ■ .,     '  ^ 

ponds  and  streams  for  its  food;  and  the  graceful  and  rapid 
darting  of  the  full  fledged  dragon-fly  as  it  chases  its  prey 
through  the  air.  The  transition  between  these  two  stages 
is  very  abrupt.  When  ready  for  its  final  moult  the  dragon- 
fly nymph  crawls  upon  the  stem  of  some  plant  or  upon  a 
stone,  its  skin  splits  down  the  back,  and  out  comes  the  imago, 
which  needs  only  to  dry  its  wings  a  little  to  be  ready  for  its 
vita  nuova  in  the  world  of  sunshine.  The  difference  between 
the  behavior  of  the  crawling,  gnawing  caterpillar  and  the 
active  honey  sucking  butterfly;  of  the  helpless  wriggling 
grub  and  the  honey  bee;  of  the  free  swimming  larva  and  the 
worm  that  burrows  in  the  sand  of  the  seashore  are  instances, 
out  of  thousands  that  might  be  given,  of  the  great  differences 
in  instinctive  behavior  at  different  periods  of  life  in  forms 
which  undergo  marked  metamorphoses  in  structure. 

Where  animals  are  hatched  in  the  form  of  the  adult  we  find 
little  change  in  instinct.  The  young  trap  door  spider,  accord- 
ing to  Moggridge,  constructs  its  tiny  tubular  dwelling  with  its 
ingeniously  fitted  trap  door  in  almost  a  perfect  miniature  of 
the  adult  nest.  Montgomery  finds  that  the  young  of  the 
orbweavers  Epeira  scolpetaria  and  E.  mdrmorea  spin,  at  their 
very  first  attempt,  a  diminutive  web  of  the  same  degree  of 
perfection  as  that  of  the  full  grown  spider.  Here,  as  in  the 
case  of  the  amphipod  previously  described,  the  young  closely 
resemble  the  older  individuals.  Where  the  change  of  form 
is  greater,  as  in  insects  with  a  gradual  or  incomplete  meta- 
morphosis, there  is  a  gradual  change  of  instinct.  The  be- 
havior of  the  tadpole  graduates  insensibly  into  the  very 


INSTINCT  99 

different  behavior  of  the  frog.  But  with  abrupt  structural 
changes  such  as  occur  in  insects  with  complete  metamor- 
phosis the  changes  in  instinct  at  successive  periods  is  equally 
great  and  often  more  striking. 

The  transitoriness  of  many  instincts  has  been  illustrated 
in  some  of  the  cases  referred  to,  in  which  the  instincts  of 
larval  life  are  superseded  by  those  of  a  later  period.  The 
same  trait  is  commonly  manifested  in  the  behavior  of  higher 
forms.  Here  the  instinct  may  be  fostered  and  continued  \ 
by  habit,  and  if  it  does  not  become  aroused  by  the  appropri-  , 
ate  objects  soon  fades  away.  According  to  Spaulding,  "A 
chicken  that  has  not  heard  the  call  of  the  mother  until 
eight  or  ten  days  old  then  hears  it  as  if  it  heard  it  not.  I 
regret  to  find  that  on  this  point  my  notes  are  not  so  full  as 
I  could  wish,  or  as  they  might  have  been.  There  is,  however, 
an  account  of  one  chicken  that  could  not  be  returned  to  the 
mother  when  ten  days  old.  The  hen  followed  it  and  tried 
to  entice  it  in  every  way;  still  it  continually  left  her  and  ran 
to  the  house  or  to  any  person  of  whom  it  caught  sight.  This 
it  persisted  in  doing,  though  beaten  back  with  a  small  branch 
dozens  of  times,  and,  indeed,  cruelly  maltreated."  If 
calves  are  prevented  from  sucking  for  some  time  after 
birth  they  frequently  although  not  invariably  lose  the 
instinct  to  suck,  and  may  then  be  safely  returned  to  the 
mother.  There  is  obviously  an  adaptiveness  in  this  transi- 
toriness of  instinct  in  higher  forms.  WTiere  the  instinct 
finds  no  proper  object  to  call  it  forth  it  is  rather  of  advantage 
to  the  animal  to  be  rid  of  it;  the  ground  is  in  a  measure 
cleared  for  the  development  of  new  adaptations. 

Modern  literature  on  animal  behavior  has  much  to  say 
regarding  the  kinship  of  instinct  and  reflex  action.  Both 
are  based  on  inherited  organization;  both  consist  to  a  con- 
siderable degree  of  purposive  actions  in  relation  to  outer 


100  INSTINCT 

objects,  although  without  a  knowledge  of  the  end  they  sub- 
serve. There  is  so  complete  a  gradation  of  responses 
between  simple  reflexes  and  complex  instincts  that  it  be- 
comes an  arbitrary  matter  where  the  line  is  drawn  between 
them.  In  ourselves  coughing,  sneezing,  winking,  hiccough- 
ing, swallowing,  vomiting,  jerking  back  when  tickled  or 
painfully  stimulated  are  commonly  set  down  as  reflexes. 
Sucking,  biting,  chewing,  spitting  out,  making  a  face  over 
disagreeable  objects,  grasping  with  fingers  and  toes,  carrying 
objects  to  the  mouth,  etc.,  are  usually  classed  as  human 
instincts  (Preyer).  These  acts  are  manifested  by  the  human 
infant  at  a  very  early  period  and  in  much  the  same  way  by 
different  individuals,  and  there  can  be  no  doubt  that  their 
relation  to  the  inherited  organization  is  the  same  as  in  the 
lower  animals.  Chewing,  spitting  out  and  making  a  face 
over  a  disagreeable  taste  are  little  more  complex  than  the 
reflexes  of  swallowing  and  coughing.  If  not  performed 
involuntarily,  there  is  at  least  a  strong  involuntary  pro- 
clivity to  their  performance  which  would  express  itself  in 
action  if  not  suppressed  by  an  effort  of  the  will.  Swallowing, 
coughing  and  sneezing  are  likewise  capable  of  voluntary 
suppression,  so  that  we  cannot  separate  these  activities 
sharply  on  the  basis  of  their  relation  to  the  will  any  more 
than  on  the  ground  of  complexity. 

In  man  the  gradation  from  the  simple  to  the  more  com- 
plex manifestations  of  instinct  is  not  so  obvious  owing  to  the 
fact  that  human  instincts  are  so  closely  interwoven  with 
habits  and  the  workings  of  intelligence;  but  in  lower  forms 
where  intelligence  is  reduced  to  a  minimum  the  relation  is 
shown  very  clearly.  In  an  animal  such  as  the  crayfish  the 
relation  of  instinct  and  reflex  action  may  be  studied  very 
advantageously  by  the  experimental  method.  The  cray- 
fish has  a  number  of  well  defined  instinctive  reactions  such. 


INSTINCT  101 

as  locomotion  by  walking  and  swimming,  darting  back  upon 
the  approach  of  danger,  seeking  dark  and  protected  situ- 
ations, rearing  up  when  threatened  and  holding  the  claws 
in  a  position  for  defence,  withdrawing  movements,  moving 
toward  certain  odors  and  feeling  about  with  the  chelae  for 
food,  seizing  food  in  the  chelae  and  passing  it  to  the  mouth, 
chewing  and  swallowing,  rejecting  objects  from  the  mouth, 
and  a  number  of  others.  The  crayfish  may  form  associa- 
tions to  a  limited  degree,  but  if  it  had  to  rely  entirely  on  its 
congenital  endowment  of  instincts  it  would  probably  get 
through  the  world  almost  as  successfully  as  it  does  with  its 
modicum  of  intelligence. 

The  relation  of  instincts  and  reflexes  in  the  crayfish  has 
been  studied  with  considerable  thoroughness  by  means  of 
operations  on  the  nervous  system.  The  nervous  mechanism 
of  the  crayfish  consists  of  a  brain  which  gives  branches  to 
the  eyes,  first  and  second  antennae,  and  anterior  part  of  the 
thorax;  a  ventral  nerve  cord  consisting  of  a  double  chain  of 
ganglia  which  is  connected  with  the  brain  by  commissures 
passing  around  the  esophagus;  and  a  small  visceral  system. 
The  ganglia  of  the  ventral  nerve  cord  are  connected  by 
cross  commissures  as  wxll  as  the  longitudinal  ones  which 
form  the  larger  part  of  the  nerve  chain,  and  they  give  off 
nerves  which  are  distributed  to  the  segments  in  which 
they  lie.  Typically  there  is  a  pair  of  ganglia  in  each  seg- 
ment of  the  body,  but  anteriorly  the  ganglia  belonging  to 
the  segments  bearing  the  mouth  parts  have  become  fused 
into  a  single  sub-esophageal  ganglion  which  supplies  these 
appendages.  The  brain  may  be  regarded  as  a  nerve  center 
homologous  with  the  ganglia  of  the  ventral  nerve  cord,  but 
like  the  subesophageal  ganglion,  it  is  formed  of  more  than 
one  pair  of  ganglia  which  have  been  fused  together. 

Experiments  on  the  crayfish  show  very  clearly  that  the 


102  INSTINCT 

instincts  of  the  animal  are  by  no  means  monopolized  by  the 
brain,  but  that  the  various  ganglia  of  the  ventral  nerve  cord 
are  the  controlling  centers  of  many  activities.  If  we  cut 
the  commissures  connecting  the  brain  with  the  chain  of 
ventral  ganglia,  the  reactions  of  the  eyes  and  antennse  take 
place  in  the  usual  way.  If  the  eye  stalk  is  stimulated  it  is 
vathdrawn;  if  the  antennules  or  antennse  are  touched  they 
are  drawn  back,  but  there  is  no  reaction  from  the  legs.  On 
the  other  hand,  if  a  leg  is  seized  an  effort  is  made  to  with- 
draw it;  if  this  is  not  successful  other  legs  may  be  employed 
to  push  against  one's  hand  or  the  chelipeds  may  reach  over 
and  pinch  the  offender.  The  crayfish  can  walk  in  the  usual 
manner,  and  when  placed  in  the  water  it  can  swim  as  well 
as  a  normal  individual.  Its  movements  are  more  restless 
than  before,  owing  to  the  lack  of  inhibitory  impulses  which 
under  ordinary  circumstances  are  issued  from  the  brain. 
Food  is  seized  by  the  chelipeds,  passed  to  the  mouth  parts, 
chewed  and  swallowed;  if  stones  or  other  innutritions  objects 
are  presented  to  the  mouth  parts  they  are  at  once  rejected, 
showing  that  connection  with  the  brain  is  by  no  means 
necessary  for  the  proper  discrimination  of  food. 

If  the  commissures  are  cut  farther  back,  between  the  sub- 
esophageal  and  the  first  thoracic  ganglia,  power  of  moving 
the  legs  still  remains,  although  locomotion  is  somewhat 
impeded.  If  a  leg  is  seized  it  is  withdrawn  or  defended  by 
the  other  appendages.  Pieces  of  meat  or  paper  given  to  the 
chelipeds  are  passed  from  one  to  the  other  and  pressed 
between  the  mouth  parts  where  they  may  be  seized  and 
swallowed.  Often  the  mouth  parts  are  tardy  in  responding, 
and  the  chelipeds  may  vainly  persist  for  hours  in  pressing 
the  object  against  them.  Sometimes  bits  of  food  are  torn 
to  pieces  and  then  offered  to  the  mouth  parts.  Stones  or 
other  hard  objects  are  not  passed  to  the  mouth,  and  if  seized 


INSTINCT  103 

are  soon  rejected.  The  legs  are  almost  incessantly  engaged 
in  cleaning  movements,  picking  at  one  another,  and  at  the 
abdomen  and  its  appendages. 

With  the  nerve  cord  cut  between  the  first  and  second 
thoracic  ganglia  the  responses  of  the  parts  in  front  of  the  cut 
are  little  affected,  but  those  of  the  last  four  pairs  of  thoracic 
legs  are  much  reduced  in  vigor.  The  chelse  still  perform 
defensive  movements,  but  the  feeding  movements  no  longer 
occur.  Cutting  the  nerve  cord  further  back  interferes  still 
more  with  the  power  of  coordinated  locomotion,  although 
the  withdrawing  and  defensive  movements  still  persist.  In 
fact,  any  pair  of  legs  will  perform  these  movements  if  the 
cord  is  cut  both  in  front  of  and  behind  the  ganglion  supply- 
ing these  legs  with  nerves. 

Each  ganglion  is  a  reflex  center  regulating  the  movements 
of  the  appendages  of  the  segment  in  which  it  lies.  Into  it 
impulses  pass  from  the  appendages  and  are  sent  out  to 
muscles  which  effect  the  withdrawing  or  the  defensive  acts 
in  response  to  the  outer  stimulus.  When  several  ganglia 
are  joined  together,  the  impulses  from  the  various  append- 
ages are  coordinated;  instead  of  a  single  adaptive  reflex,  we 
have  a  complex  cooperative  response,  such  as  occurs  in 
walking,  cleaning  movements  and  mutual  defense^of  ap- 
pendages which  are  seized.  What  a  wonderful  combination 
and  coordination  of  impulses !  From  the  simple  reflex  of  the 
isolated  segmental  ganglion  to  the  complex  behavior  of  the 
brainless  crayfish,  and  from  this  to  the  still  more  complex 
behavior  of  the  normal  animal  there  is  a  regular  gradation. 
Nowhere  can  we  draw  a  sharp  line  between  reflex  behavior 
on  the  one  hand  and  instinct  on  the  other.  Both  are  based 
on  a  wonderfully  complex  and  beautifully  organized  nervous 
mechanism. 

Among  the  most  remarkable  of  the  instincts  of  crusta- 


104  INSTINCT 

ceans  is  the  peculiar  trait  shown  by  several  species  of  spider 
crabs  of  decking  themselves  out  with  a  covering  of  seaweed, 
sponges,  hydroids,  etc.,  so  that  they  are  quite  effectually 
concealed  in  their  natural  surroundings.  Spider  crabs 
kept  in  aquaria  have  been  seen  by  several  observers  to  snip 
off  bits  of  seaweed  with  their  pincers,  reach  over  their  backs 
and  stick  them  down  among  the  stiff  curved  hairs  which 
occur  on  the  upper  surface  of  the  carapace.  The  seaweeds 
and  sessile  animals  thus  transplanted  often  grow,  so  that 
the  back  of  the  crab  comes  to  be  a  veritable  botanical  and 
zoological  garden  on  a  small  scale.  There  are  few  instinc- 
tive acts  which  appear  to  be  more  the  result  of  deliberate 
intention,  yet  the  disguisement  is  not  only  instinctive,  but, 
as  Minkiewicz  has  shown,  it  is  performed  by  crabs  in  which 
the  brain  is  entirely  cut  off  from  connection  with  the  legs  by 
cutting  the  esophageal  commissures  that  lead  to  the  large 
ventral  ganglion.  The  sense  of  wonder  with  which  instinct 
was  formerly  regarded,  and  which  the  reflex  theory  might 
seem  to  rudely  destroy,  is  a  sentiment  which  can  scarcely 
fail  to  arise  when  we  contemplate  the  organization  which 
makes  possible  the  blind  performance  of  such  remarkable  acts. 
Among  the  insects,  as  in  the  Crustacea,  the  seat  of  many 
instincts  is  in  the  ventral  ganglia  rather  than  in  the  brain. 
A  decapitated  fly  is  able  to  walk  and  fly  and  go  through  with 
elaborate  movements  of  cleaning  the  wings,  legs  and  body. 
In  experiments  made  by  the  writer  on  the  water  scorpion, 
Ranatra,  it  was  found  that  after  decapitation  the  insects 
were  able  to  walk  and  swim  almost  as  well  as  before.  In 
fact,  they  became  much  more  restless  and  would  walk  about 
for  hours  without  *coming  to  rest;  and  when  they  finally 
became  quiet  they  could  be  aroused  by  the  slightest  stimuli. 
When  placed  on  their  backs  they  would  readily  right  them- 
selves.   If  the  tip  of  the  breathing  tube  was  seized  while  the 


INSTINCT  105 

insect  was  swimming  its  efforts  to  swim  away  would  be 
performed  with  much  greater  vigor.  If  it  does  not  effect 
its  escape  by  this  method  it  has  recourse  to  a  remarkably 
neat  and  apparently  intelligent  device.  The  hind  legs  are 
thrown  back  as  far  as  possible,  whereby  they  are  enabled 
to  grasp  the  breathing  tube  a  short  distance  behind  the 
body;  then  by  exerting  a  pull  they  bend  the  body  ven- 
trally.  This  soon  places  the  second  pair  of  legs  so  that  the 
offending  object  can  be  reached,  when  all  four  legs  are  em- 
ployed to  push  the  body  away,  which  is  very  frequently 
accomplished.  The  behavior  of  a  decapitated  Ranatra  in 
this  situation  certainly  affords  an  excellent  simulation,  not 
only  of  purposive  action,  but  also  of  considerable  ingenuity 
in  its  accomplishment. 

The  reactions  of  the  brainless  frog  form  the  stock  illus- 
trations of  reflex  action.  The  withdrawal  of  a  foot  when 
pinched  is  one  of  the  simplest  of  these.  When  a  drop  of 
acid  is  placed  on  one  side  of  the  body  the  hind  foot  of  that 
side  is  brought  forward  to  wipe  it  off.  With  a  somewhat 
stronger  stimulus  the  fore  leg  of  the  same  side  may  be 
moved  back  to  the  irritated  spot.  If  the  acid  is  placed  on 
the  middle  of  the  posterior  part  of  the  back  both  hind  legs 
are  employed  to  remove  it.  We  have  here  reflexes  of  a 
higher  degree  of  complexity  involving  the  coordinated  move- 
ments of  many  muscles.  If  a  frog  with  the  greater  part  of 
its  brain  removed  is  taken  in  the  hands  it  uses  both  hind 
legs  to  push  against  the  hands,  and  at  the  same  time  inflates 
the  lungs  with  air,  causing  the  body  to  swell  so  that  it  more 
readily  slips  from  the  grasp.  The  use  of  the  hind  limbs 
and  the  swelling  of  the  body  may  be  regarded  as  two  com- 
plex refle  es  excited  by  the  same  cause  and  which  cooperate 
to  enable  the  animal  to  effect  its  escape,  but  the  behavior 
may  equally  well  be  described  as  an  instinctive  reaction. 


106  INSTINCT 

The  clasping  of  the  female  frog  by  the  male  during  the 
breeding  season  affords  a  typical  example  of  instinctive 
behavior;  nevertheless,  it  occurs  in  entire  independence  of 
the  higher  nerve  centers.  "The  Abbe  Spallanzani  showed 
tliat  a  male  frog  may  have  its  head  cut  off  during  copulation 
without  ceasing  to  cling  tenaciously  to  the  female.  Goltz 
went  still  further  and  cut  off  the  head  of  a  male,  then  cut  the 
body  through  between  the  third  and  fourth  vertebrae,  and 
removed  the  viscera  from  the  body  cavity;  the  section  of  the 
frog  that  remained  after  these  operations  consisted  of  the 
first  three  vertebrae,  the  pectoral  girdle  and  the  fore  legs. 
Yet  when  the  skin  of  the  inner  surfaces  of  the  fore  legs  was 
rubbed  with  the  finger  this  segment  would  show  the  same 
clasping  efforts  as  a  normal  male  frog." 

Nearly  all  the  characteristic  responses  of  the  frog  will  take 
place  in  individuals  deprived  of  the  cerebral  hemispheres 
which  are  the  part  of  the  brain  usually  considered  as  the 
seat  of  intelligence  and  volition.  Such  a  frog,  if  given  suffi- 
cient time  to  recover  from  the  shock  of  the  operation,  will 
leap  about  and  swim  spontaneously,  snap  at  insects  which 
come  within  range,  bury  itself  in  the  mud  on  the  approach 
of  winter,  and  in  many  other  ways  behave  in  a  normal 
ranine  manner. 

Beginning  with  the  anterior  part  of  the  brain  and  destroy- 
ing successively  the  parts  of  the  central  nervous  system 
lying  behind  it,  we  cause,  one  after  the  other,  the  various 
instinctive  acts  of  the  frog  to  disappear,  until  we  have  left 
only  the  reflexes  of  the  posterior  part  of  the  spinal  cord.  We 
reduce  the  frog  to  a  more  and  more  simple  type  of  reflex 
mechanism,  but  we  cannot  say  where  the  animal  ceases  to 
be  more  than  a  reflex  mechanism  of  a  complicated  kind. 
The  behavior  of  the  frog  is  almost  entirely  made  up  of  in- 
stinctive and  reflex  acts,  many  of  which  have  their  seat 


INSTINCT  107 

either  in  the  spinal  cord  or  the  lower  centers  of  the  brain. 
It  thus  resembles  the  segmental  reflexes  which  constitute 
much  of  the  behavior  of  worms  and  arthropods.  In  the 
frog,  as  Schrader  has  observed,  the  central  nervous  system 
''can  be  divided  into  a  series  of  sections  each  of  which  is  capa- 
ble of  performing  an  independent  function.''  Its  mode 
of  action  is  hence  of  the  same  fundamental  kind  that  we 
find  in  the  nervous  systems  of  the  lower  articulate  animals. 

The  relation  of  reflex  action  to  instinct  which  is  disclosed 
through  operations  on  the  nervous  system  is  shown  also 
by  a  study  of  the  gradual  development  of  instinct  through 
the  animal  kingdom.  The  behavior  of  the  protozoa,  as  we 
have  seen,  consists  mostly  of  rather  simple  stereotyped 
activities  which  have  all  the  directness  of  the  simple  reflex 
acts  of  higher  forms.  The  behavior  of  the  lower  Metazoa 
falls  largely  within  the  same  general  type.  Tracing  the 
evolution  of  behavior  upward  we  find  a  gradual  increase  in 
the  number,  complexity  and  perfection  of  reflex  acts. 
Where  instinct  may  be  said  to  begin  is  an  arbitrary  matter. 
If  instinct  be,  as  Spencer  defines  it,  "compound  reflex  ac- 
tion," it  begins  of  course  where  reflex  action  passes  from  the 
simple  to  the  compound,  but  this  point  is  not  so  easy  to 
mark  as  theoretically  it  might  appear.  It  is  commonly  said 
that  in  reflex  action  only  a  part  of  the  body  responds,  as  in 
winking  the  eye,  or  jerking  back  the  foot,  whereas  in  instinc- 
tive behavior  there  is  a  response  by  the  organism  as  a  whole. 
This  distinction  is  at  times  difficult  to  draw,  and  it  is  not 
consistently  adopted  by  most  writers,  but  it  is  perhaps  as 
useful  a  distinction  as  can  be  made. 

While  instinct  is  most  intimately  related  in  its  nature  and 
origin  to  reflex  action  it  would  be  an  error  to  regard  it  as  con- 
sisting of  nothing  but  direct  responses  to  external  stimuli. 
The  animal  is  not  merely  a  machine  responding  to  the 


108  INSTINCT 

various  influences  from  the  environment  which  affect  it. 
It  possesses  a  native  fund  of  impulse  which  causes  it  to  act 
in  more  or  less  definite  ways  independently  of  the  stimula- 
tions of  the  outer  world.  Several  modern  writers  have 
over-emphasized  the  element  of  responsiveness  in  instinct, 
as  if  an  animal  were  like  an  instrument  played  upon  by 
outer  forces  and  had  its  actions  fatally  determined  by  the 
action  of  those  forces  on  its  own  inner  mechanism.  Other 
writers  have  treated  instinct  as  determined  by  a  sort  of 
internal  impulsion.  The  latter  conception  is  implied  in  the 
German  word  "Trieb,"  or  driving  force,  and  in  Paley's 
''propensity  prior  to  experience."  Lloyd  Morgan  says  in 
speaking  of  instinct:  "Initiated  by  an  external  stimulus 
or  group  of  stimuli,  it  is  at  any  rate  in  many  cases,  deter- 
mined also  in  greater  degree  than  reflex  action  by  an  internal 
factor  which  causes  uneasiness  or  distress,  more  or  less 
marked,  if  it  do  not  find  its  normal  instinctive  satisfactions. 
Take,  for  example,  the  before  mentioned  instinct  of  the 
great  water-beetle  to  leave  the  pond  and  burrow  in  the  bank 
when  the  time  for  pupation  is  at  hand.  There  is  something 
more  here  than  a  local  response  to  an  external  stimulus; 
something  more,  it  would  seem,  than  mere  reflex  action. 
There  are  activities  affecting  the  whole  behavior  of  the 
organism,  and  there  seem  to  be  internal  promptings  of  some 
kind  due  to  organic  conditions  whose  seat  is  in  the  body  of 
the  developing  larva.  Or  take  the  migration  of  birds,  their 
nest-building  instincts,  the  activities  involved  in  the  rearing 
of  their  young;  there  is  surely,  it  may  be  said,  something  in 
all  this  which  may  be  distinguished,  even  if  the  line  of  de- 
markation  be  hard  to  draw,  from  reflex  action.  We  cannot 
say  more,  however,  than  that  the  one  is  a  more  fully  cor- 
porate act  than  the  other.'' 
It  is  without  question  that  internal  states  form  the  prompt- 


INSTINCT  109 

ings  of  many  instinctive  acts.  Hunger  drives  the  lioness 
to  seek  for  prey;  sexual  impulses  lead  to  the  search  for  mates; 
and  a  bird  in  confinement  may  become  uneasy  when  the 
time  for  migration  arrives.  The  same  thing  is  even  more 
conspicuously  illustrated  in  the  instinct  of  play.  A  lamb 
may  frisk  about  from  sheer  good  feeling.  A  kitten  may 
crouch  and  spring  as  if  upon  a  mouse  when  there  is  no  ex- 
ternal object  to  excite  its  action.  And  where  the  play 
activities  are  associated  with  external  objects,  the  latter 
serve  only  to  awaken  the  stored  energy  of  impulse  which 
may  be  nearly  ready  to  discharge  on  its  own  account.  The 
play  impulse  which  may  sometimes  vent  itself  in  random 
movements  usually  takes  fairly  definite  channels  of  expres- 
sion which  are  quite  characteristic  of  particular  species  of 
animals.  The  energies  of  the  young  animal  tend  to  dis- 
charge themselves  in  movements  similar  to  those  which  form 
the  regular  behavior  of  the  adult,  and  a  certain  degree  of 
proficiency  is  reached  in  those  activities  which  form  the 
more  serious  occupations  of  later  life.  But  the  promptings 
to  such  behavior  are  due  mainly  or  wholly  to  internal 
impulses. 

The  element  of  internally  initiated  impulse  in  instinct  is 
not  confined  to  higher  forms.  It  is  probably  coextensive 
with  animal  life.  Amid  all  the  stereotyped  responses  of 
the  Protozoa  we  have  a  large  element  of  activity  determined 
by  internal  factors.  The  almost  constant  swimming  of 
many  infusorians,  and  the  regular  rhythmical  activity  of 
others  are,  like  the  beating  of  the  heart  and  other  organic 
rhythms,  the  result  of  causes  within  the  organism. 

It  is  of  course  difficult  in  many  cases  to  ascertain  whether 
activity  results,  perhaps  indirectly,  from  outer  stimulations 
or  from  internal  changes.  In  a  great  many  cases  the 
organism  needs  but  a  slight  provocation  to  discharge  its 


no  INSTINCT 

energies  in  instinctive  acts.  When  the  spider  spins  its  web, 
when  the  wasp  digs  a  hole  and  stores  it  with  a  certain  kind 
i  of  prey  for  its  young,  and  when  the  bird  builds  its  nest,  and 
/  the  beaver  its  dam  there  is  of  course  response  to  certain 
features  of  the  environment;  there  is  also  an  innate  propen- 
sity for  the  organic  machinery  to  work  in  certain  ways, 
much  as  a  piece  of  clock  work  runs  in  a  particular  fashion 
after  it  has  been  wound  up  and  set  going. 

Activity  which  is  internally  initiated  is  not  fundament- 
ally different  from  activity  which  we  commonly  call  reflex; 
the  stimuli  by  which  it  is  evoked  are  internal  instead  of 
external;  they  result  in  many  cases  from  the  rhythms  of 
organic  functions,  chance  discharges  of  nervous  energy 
due  to  various  physiological  changes,  and  various  other 
factors.  Such  activities  are  to  a  high  degree  characteristic 
of  particular  species  and  are  doubtless  as  rigidly  determined 
by  organization  as  are  the  direct  responses  to  external 
stimuli. 

The  nature  of  the  instinctive  act  that  may  be  performed 
in  a  given  situation  is  notoriously  dependent  upon  the 
internal  condition  of  the  animal.  The  same  stimulus  may 
evoke  in  different  states  quite  contrary  impulses.  The 
sight  and  smell  of  food  may  arouse  an  animal  to  vigorous 
efforts  to  secure  it,  or  produce  feelings  of  aversion  and 
movements  of  avoidance,  according  to  the  creature's  state 
of  hunger  or  satiety.  The  sexual  behavior  of  animals  is 
dependent  to  a  very  marked  degree  upon  internal  conditions 
which  are  correlated  with  the  production  and  maturation 
of  the  sex  cells.  Salmon  begin  their  up-stream  migrations, 
the  male  frog  develops  his  tendency  to  clasp  the  female; 
birds  herald  the  advent  of  the  breeding  season  with  court- 
ship and  song,  and  the  males  of  many  mammals  show  at 
this  time  an  unusual  degree  of  belligerency.    The  change 


INSTINCT  111 

in  instinctive  behavior  during  the  breeding  season  may  be 
due  to  the  production  of  internal  secretions  which  influence 
the  irritabiUty  of  certain  parts  of  the  nervous  system,  but, 
however  caused,  it  is,  like  the  varying  responses  to  food, 
water,  etc.,  pretty  closely  subservient  to  the  needs  of  the 
species. 

This  dependence  of  behavior  upon  internal  conditions 
naturally  increases  the  range  of  its  possible  adaptations. 
Animals  are  often  endowed  with  adaptive  responses  cor- 
responding to  this,  that,  or  the  other  internal  state.  Pre- 
vious exercise  and  many  other  factors  change  these  internal 
states,  so  that  what  an  animal  may  do  in  a  given  situation 
is  not  to  be  inferred  from  the  external  conditions  alone.  If 
one  response  does  not  suit  the  animal  tries  another,  and  so 
on.  The  condition  of  the  animal  is  changed  after  one  or 
more  reactions  and  this  change  produces  a  different  reaction 
to  the  stimulus. 

According  to  Whitman,  the  leech  Clepsine  when  it  is 
stimulated  may  roll  into  a  ball,  hug  the  bottom,  or  crawl 
away.  "If  the  leech  has  eggs  it  will  not  roll  up,  but  if  it 
has  no  eggs,  or  if  it  has  young,  it  may  adopt  either  mode  of 
escape,  while  if  it  has  eggs  it  has  no  choice  but  to  remain 
quiet  over  them.  The  act  of  rolling  up  into  a  passive  ball 
may  be  performed  (a)  under  compulsion,  as  when  it  is  her 
last  resort  in  self  defense;  (b)  under  a  milder  provocation, 
as  one  of  three  courses  of  behavior,  as  when  the  resting  place 
is  turned  up  to  light,  and  the  choice  is  offered  between 
remaining  quiet  in  place,  creeping  away  at  leisure,  or  rolling 
into  a  ball  and  dropping  to  the  bottom;  (c)  or  finally,  under 
no  special  external  stimulus,  but  rather  from  internal  motive, 
the  normal  demand  for  rest  and  seclusion,  presumably  very 
strong  in  Clepsine  after  gorging  itself  with  the  blood  of 
its   turtle  host."     ''The  differential  reaction,"  says  Lloyd 


112  INSTINCT 

Morgan,  "according  as  the  animal  has  eggs  or  not  suggests 
intelligence;  but,"  he  adds — and  this  seems  to  me  to  be  a 
more  probable  conclusion — that  "it  may  be  instinct  varying 
according  to  the  conditions  of  stimulation." 

Varied  activity  under  unfavorable  conditions  charac- 
terizes alike  the  behavior  of  the  Protozoa  and  the  most 
highly  evolved  animals.  While  not  involving  intelligence 
it  performs,  in  a  measure,  the  function  of  intelligence,  as  it 
gives  the  animal  greater  opportunities  for  making  favorable 
adjustments.  ^^ Nature,^  says  James,  ^^ implants  contrary 
impulses  to  act  in  many  classes  of  things,  and  leaves  it  to 
slight  alterations  of  the  conditions  of  the  individual  case  to 
decide  which  impulse  shall  carry  the  day.  Thus  greediness 
and  suspicion,  curiosity  and  timidity,  coyness  and  desire, 
bashfulness  and  vanity,  sociability  and  pugnacity  seem  to 
shoot  over  into  each  other  as  quickly,  and  to  remain  in  as 
unstable  equilibrium,  in  the  higher  birds  and  mammals  as 
in  man.  They  are  all  impulses,  completely  blind  at  first, 
and  productive  of  motor  reactions  of  a  rigorously  deter- 
minate sort.  Each  of  them,  tlien,  is  an  instinct,  as  instincts 
are  commonly  defined.  But  they  contradict  each  other — 
'experience'  in  each  particular  opportunity  of  application 
usually  deciding  the  issue.  The  animal  that  exhibits  them 
loses  the  instinctive  demeanor  and  appears  to  lead  a  life  of 
hesitation  and  choice,  an  intellectual  life,  not,  howevevy 
because  lie  has  no  instincts — ^rather  because  he  has  so  many 
of  them  that  they  block  each  other's  path." 

At  its  first  appearance  intelligence  is  able  to  modify  but 
slightly  the  course  of  instinctive  behavior.  It  is  a  faculty 
which,  as  Hobhouse  remarks,  "  arises  within  the  sphere  of 
instinct "  and  is  devoted  to  the  task  of  enabling  the  instinc- 
tive proclivities  of  the  animal  to  work  themselves  out  more 
effectively.    The  close  connection  of  intelligence  and  instinct 


INSTINCT  113 

is  shown  by  the  fact  that  animals  are  so  exceedingly  stupid 
in  everything  not  closely  related  to  their  instinctive  in- 
terests. A  cat  pays  not  the  least  attention  to  a  multitude 
of  things  going  on  around  her,  but  the  sight  of  a  canary  or 
the  noise  made  by  a  gnawing  mouse  puts  her  on  the  qui-vive. 
Only  a  few  objects  have  meaning;  the  rest  do  not  form  a 
part  of  what  might  be  called  her  effective  environment; 
to  her  they  are  non-existent. 

Not  only  in  lower  forms,  but  in  the  higher  members  of  the 
animal  kingdom  as  well,  intelligence  may  be  said  to  be  the 
handmaid  of  instinct.  Animals  profit  by  experience  in 
order  to  live  their  life  along  the  lines  marked  out  for  them 
by  their  instinctive  make-up,  and  whatever  pleasure  or 
satisfaction  their  lives  may  bring  is  attained  by  following 
their  instinctive  bent.  "Why,"  asks  James,  in  a  significant 
passage,  "do  the  various  animals  do  what  seem  to  us  such 
strange  things  in  the  presence  of  such  outlandish  stimuli? 
Why  does  the  hen,  for  example,  submit  herself  to  the  tedium 
of  incubating  such  a  fearfully  uninteresting  set  of  objects  as 
a  nestful  of  eggs,  unless  she  have  some  prophetic  inkling 
of  the  result?  The  only  answer  is  ad  hominem.  We  can 
only  interpret  the  instincts  of  brutes  by  what  we  know  of 
instincts  in  ourselves.  Why  do  men  always  lie  down,  when 
they  can,  on  soft  beds  rather  than  on  hard  floors?  Why 
do  they  sit  around  the  stove  on  a  cold  day?  .  .  .  Why 
does  the  maiden  interest  the  youth  so  that  everything  about 
her  seems  more  important  and  significant  than  anything 
else  in  the  world?  Nothing  more  can  be  said  than  that  these 
are  human  ways,  and  that  every  creature  likes  its  own  ways, 
and  takes  to  following  them  as  a  matter  of  course.  Science 
may  come  and  consider  these  ways  and  find  that  most  of 
them  are  useful.  But  it  is  not  for  the  sake  of  their  utility 
that  they  are  followed,  but  because  in  following  them  we 


114  INSTINCT 

feel  that  it  is  the  only  appropriate  and  natural  thing  to 
do.  .  .  .  And  so,  probably,  does  each  animal  feel  about  the 
particular  things  it  tends  to  do  in  the  presence  of  particular 
objects.  ...  To  the  lion  it  is  the  lioness  which  is  made  to 
be  loved;  to  the  bear,  the  she-bear.  To  the  broody  hen  the 
notion  would  probably  seem  preposterous  that  there  should 
be  a  creature  in  the  world  to  whom  a  nestful  of  eggs  was  not 
the  utterly  fascinating  and  precious,  never-to-be-too-much- 
sat-upon-object  which  it  is  to  her." 

With  increasing  intelligence  attention  becomes  devoted 
to  things  which  are  more  remotely  connected  with  instinctive 
interests,  but  even  in  ourselves  instinct,  in  one  form  or 
another,  supplies  the  basis  of  most  of  our  springs  of  action. 
With  us,  as  with  the  lower  animals,  self-preservation  and 
the  perpetuation  of  the  stock  afford,  broadly  interpreted, 
the  main  business  of  life. 

BIBLIOGRAPHY 
Darwin,  C.    Origin  of  Species,  1859.    Posthumous  essay  on  Instinct 

published  in  Romanes*  Mental  Evolution  in  Animals. 
HoBHOUSE,  L.  T.     Mind  in  Evolution,  '01. 

Hudson,  W.  H.     The  Naturalist  in  the  La  Plata.     London,  '95. 
James,  W.    Principles  of  Psychology,  2  vols.  '90. 
Morgan,  C.  L.    Habit  and  Instinct,  London,  '96.    Animal  Behaviour, 

London,  '00. 
Reimarus,  H.  S.    AUgemeine  Betrachtungen  liber  die  Triebe  der 

Thiere.     Hamburg,  1773,  3d  ed. 
Romanes,  G.  J.     Animal  Intelligence,  N.  Y.,  '83.     Mental  Evolution 

in  Animals,  London,  '85. 
Schneider,  G.  H.     Der  thierische  Wille,  Leipzig,  '80. 
Schneider,  K.  C.     Vorlesungen  iiber  Tierpsychologie,  '09. 
Spaulding,  D.  a.     Instinct,  With  Original  Observations  on  Young 

Animals.     Macmillan's  Mag.  27,  282,  '73.     Reprinted  in  Pop. 

Sci.  Mon.  61,  126,  '02. 
Spencer,  H.     Principles  of  Psychology,  London,  1855. 
Wasmann,    E.     Instinkt    und    Intelligenz    im    Tierreich,    3d    ed., 

Freiburg,   i.    B..   '05.     Translation  of  2nd  ed.   St   Louis,  '03. 
Ziegler,  H.  E.    Der  Begriff  des  Instinktes  einstund  jetzt.   Jena,  '10. 


CHAPTER  VI 
THE  EVOLUTION  OF  INSTINCT 

"The  primary  roots  of  instincts  reach  back  to  the  constitutional 
properties  of  protoplasm,  and  their  evolution  runs,  in  general,  parallel 
with  organogeny.  As  the  genesis  of  organs  takes  its  departure  from 
the  elementary  structure  of  protoplasm,  so  does  the  genesis  of  in- 
stincts proceed  from  the  fundamental  functions  of  protoplasm." — 
Whitman,  Animal  Behavior. 

"  Instinct  precedes  intelligence  both  in  ontogeny  and  in  phylogeny, 
and  it  has  furnished  all  the  structural  foundations  employed  by 
intelligence." — Ibid. 

"  It  will  be  universally  admitted  that  instincts  are  as  important  as 
corporeal  structures  for  the  welfare  of  each  species  under  its  present 
conditions  of  life.  .  .  .  If  it  can  be  shown  that  instincts  do  vary 
ever  so  little,  then  I  can  see  no  difficulty  in  natural  selection  preserv- 
ing and  continually  accumulating  variations  of  instinct  to  any  extent 
that  was  profitable.  It  is  thus,  as  I  believe,  that  all  the  most  complex 
and  wonderful  instincts  have  originated." — Darwin,  Origin  of 
Species. 

Efforts  to  explain  the  origin  of  instinct  by  gradual  evolu- 
tion were  made  from  time  to  time  before  Darwin  applied 
his  theory  of  natural  selection  to  the  solution  of  the  problem. 
The  most  noteworthy  theory  was  Lamarck's  doctrine  that 
instinct  is  inherited  habit.  It  is  well  knowTi  that  actions 
frequently  performed  come  in  course  of  time  to  be  performed 
automatically  and  unconsciously,  as  is  illustrated  by  the 
familiar  example  of  learning  to  play  the  piano.  Granting 
that  the  modifications  produced  by  habit  are  inherited,  it 
is  evident  that  the  repetition  of  an  action  generation  after 
generation  would  produce  a  congenital  proclivity  to  its 
performance  which  might  in  time  develop  into  a  true  in- 
stinct.    Since  habits  are  so  frequently  the  result  of  intel- 

115 


116  THE  EVOLUTION  OF  INSTINCT 

ligent  experience,  instinct  was  conceived  by  some  writers  as 
due  to  the  gradual  automatizing  of  such  experience  by 
frequent  repetition;  in  the  words  of  G.  H.  Lewes,  instinct 
is  "lapsed  intelligence,"  a  view  which  makes  intelligence 
first  in  order  of  appearance  and  instinct  a  secondary  result 
of  a  sort  of  psychic  degeneration. 

As  Whitman  has  urged,  according  to  the  doctrine  of 
Lewes,  "we  should  expect  to  find  the  lowest  animals  free 
from  instinct  and  possessed  of  pure  intelligence.  In  the 
higher  forms  we  should  expect  to  see  intelligence  lapsing 
more  and  more  into  pure  instinct."  As  every  student  of 
animal  behavior  now  knows,  we  find  just  the  reverse. 
Among  low  forms  behavior  is  all  but  exclusively  of  the  reflex 
type.  Passing  up  the  animal  series  we  find  intelligence 
gradually  growing  upon  instinctive  foundations.  '^In  higher 
forms  not  a  single  case  of  intelligence  lapsing  into  instinct 
is  known.  In  forms  that  give  indubitable  evidence  of 
intelligence  we  do  not  see  conscious  reflection  crystallizing 
into  instinct,  but  we  do  find  instinct  coming  more  and  more 
under  -the  sway  of  intelligence." 

Herbert  Spencer,  who  was  keenly  alive  to  the  difficulties 
of  the  theory  of  lapsed  intelligence  as  an  explanation  of  the 
origin  of  instinct  in  general,  put  forward  an  ingenious  specu- 
lation in  which  he  attempted  to  derive  instinct  from  reflex 
action  and  the  inheritance  of  acquired  associations  between 
reflexes.  In  order  to  illustrate  how  an  instinct  might  arise 
he  takes  a  low  aquatic  creature  with  rudimentary  eyes. 
"Sensitive  as  such  eyes  are  only  to  marked  changes  in  the 
quantity  of  light,  they  can  be  affected  by  opaque  bodies 
moving  in  the  surrounding  water,  only  when  such  bodies 
approach  close  to  them.  But  bodies  carried  by  their  motion 
very  near  to  the  organism  will,  by  their  further  motion,  be 
brought  in  contact  with  it.   .  .  .    In  its  earliest  forms  sight  is. 


THE  EVOLUTION  OF  INSTINCT  117 

as  before  said,  little  more  than  anticipatory  touch;  visual  im- 
pressions are,  in  all  these  creatures,  habitually  followed  by 
tactual  ones.  But  tactual  impressions  are,  in  all  these  crea- 
tures, habitually  followed  by  contractions.  .  .  .  From  the 
zoophytes  upward  touch  and  contraction  form  an  habitual 
sequence;  and  hence,  in  creatures  whose  incipient  vision 
amounts  to  little  more  than  anticipatory  touch,  there  con- 
stantly occurs  the  succession — a  visual  impression,  a  tactual 
impression,  a  contraction."  This  habitual  association  will 
link  the  two  responses  so  that  a  contraction  will  follow 
immediately  upon  the  visual  stimulus.  The  effect  of  such 
experiences  accumulated  by  heredity  generation  after 
generation  is  to  establish  a  new  congenital  response  which 
is  of  value  to  the  species.  With  increased  powder  of  sensory 
discrimination  the  visual  stimuli  produced  by  smaller  objects 
whose  contact  does  not  cause  a  protective  contraction,  but 
rather  the  activities  of  food  taking,  may  in  a  similar  manner 
become  associated  with  movements  of  prehension,  thus 
enabling  the  animal  to  react  in  different  ways  to  objects  at  a 
distance.  In  this  way  Spencer  supposes  instincts  to  have 
been  built  up  by  growing  out  of  simple  reflex  acts  instead  of 
being  the  outcome  of  a  lapsed  intelligence.  Spencer's 
conception  is  more  congruous  with  the  general  doctrine  of 
psychic  evolution  and  does  not  involve  the  assumption  that 
the  lower  we  go  in  the  animal  kingdom  the  more  purely 
intelligent  the  actions  of  animals  become. 

The  theory  of  Spencer,  which  was  put  forward  in  1855,  is 
based  entirely  on  the  assumption  of  the  transmission  of 
acquired  characters  like  so  many  other  of  his  psychological 
speculations.  After  the  "Origin  of  Species"  was  published 
Spencer  accepted  the  theory  of  natural  selection,  but  as- 
signed to  it  a  subordinate  rdle,  especially  in  the  evolution 
of  mind. 


lis  THE  EVOLUTION  OF  INSTINCT 

The  Lamarckian  theory  of  the  origin  of  instinct  has  in 
more  or  less  modified  forms  enjoyed  a  wide  popularity  among 
writers  on  genetic  psychology.  The  veteran  psychologist 
Wundt  in  his  discussion  of  theories  of  instinct  in  his  Human 
and  Animal  Psychology  assumes  unquestioningly  the  trans- 
mission of  acquired  characters,  practically  ignoring  the 
doctrine  of  natural  selection,  and  even  representing  that 
Darwin  ''explains  instinct  as  inherited  habit"! 

One  of  the  most  extreme  positions  is  that  of  Eimer  who 
rejects  the  theory  of  natiu*al  selection  and  adopts  the  pure 
Lamarckian  standpoint.  One  of  his  arguments  in  favor  of 
this  theory  is  drawn  from  the  instincts  of  the  mason  wasp, 
Odynerus  parietum.  This  species  provisions  its  nest  with 
larvae  which  it  paralyzes  by  stinging  them  in  the  ventral 
ganglia.  After  collecting  several  larvae  and  storing  them 
in  a  hole  in  the  ground,  the  wasp  lays  an  egg  on  the  store  of 
food,  seals  up  the  hole  with  clay,  and  then  begins  the  con- 
struction of  another  nest.  "  What  a  wonderful  contrivance  " ! 
exclaims  Eimer,  ''What  calculation  on  the  part  of  the  animal 
must  have  been  necessary  to  discover  it!  The  larvae  of  the 
wasp  require  animal  food.  Dead  food  enclosed  in  the  cell 
would  soon  putrefy;  living  active  animals  would  disturb  the 
egg,  and  accordingly  the  wasp  paralyzes  grubs  and  packs 
them  like  sacks  of  meal  one  after  another  in  the  cell.  How 
did  she  arrive  at  this  habit?  At  the  beginning  she  prob- 
ably killed  larvae  by  stinging  them  anywhere  and  then 
placed  them  in  the  cell.  The  bad  results  of  this  showed 
themselves;  the  larvae  putrefied  before  they  could  serve 
as  food  for  the  larval  wasps.  In  the  meantime  the  mother- 
w\isp  discovered  that  those  larvae  which  she  had  stung  in 
particular  parts  of  the  body  were  motionless  but  still  alive, 
and  then  she  concluded  that  larvae  stung  in  this  particular 
way  could  be  kept  for  a  longer  time  unchanged  as  living 


THE  EVOLUTION  OF  INSTINCT 


119 


motionless  food In  this  case  it  is  absolutely  im- 
possible that  the  animal  has  arrived  at  its  habit  otherwise 
than  by  reflection  upon  the  facts  of  experience." 

The  careful  studies  of  the  Peckhams  on  the  instincts  of  the 
solitary  wasps  have  shown  that  many  of  the  assumptions 
upon  which  Eimer  rests  his  argument  are  erroneous.  In 
the  first  place  the  Peckhams  found  that  the  insects  stored  as 
food  were  by  no  means  uniformly  paralyzed  and  that  in 


Fig.  12. — Th£  wasp  Ammophila  stinging  a  caterpillar.     (After  Peckham. ) 

most  nests  several  caterpillars  died.  Even  where  all  were 
dead  the  wasp  larvae  fed  upon  them,  so  that  it  is  open  to 
question  if  much  is  gained  b}^  having  the  prey  in  a  paralyzed 
condition.  The  Peckhams  conclude  that  "the  primary 
purpose  of  the  stinging  is  to  overcome  resistance  and  to 
prevent  the  escape  of  the  victims,  and  that  incidentally  some 


120  THE  EVOLUTION  OF  INSTINCT 

of  them  are  killed  and  others  paralyzed.''  In  Ammophila 
'^  the  prey  may  be  stung  so  slightly  that  it  can  rear  and  strug- 
gle violently  or  so  severely  that  it  dies  almost  at  once,  and 
in  neither  case  is  a  break  made  in  the  generation  of  Ammo- 
philes,  since  in  the  former,  the  egg  or  larva  is  so  firmly  fas- 
tened as  to  keep  its  hold,  while  in  the  latter  the  dead  and 
decomposing  caterpillar  is  eaten  without  dissatisfaction  or 
injury." 

An  important  fact  which  Eimer  has  apparently  failed  to 
consider  is  that  in  most  of  the  solitary  wasps  the  nest  with 
its  provision  and  egg  is  abandoned  as  soon  as  completed  and 
never  seen  again,  the  wasp  dying  before  its  progeny  emerges. 
The  same  is  true  among  the  more  primitive  bees,  such  as 
Osmia,  where  elaborate  provision  is  made  for  the  larva 
which  only  emerges  the  following  year.  The  typical  con- 
dition among  the  solitary  species  of  both  bees  and  wasps  is 
one  in  which  the  parent  never  sees  its  own  offspring,  never 
has  an  opportunity  of  watching  the  results  of  its  own  experi- 
ments. Unless  gifted  with  a  truly  preternatural  intelli- 
gence what  means  has  a  solitary  wasp  or  bee  which  never 
sees  the  larva  or  pupa  of  its  own  species  of  knowing  what 
conditions  of  food  and  habitation  are  the  most  suitable  for 
its  progeny?  Far  from  being  the  only  possible  explanation 
of  the  origin  of  the  remarkable  instinct  he  has  cited,  the 
doctrine  of  Eimer  is  improbable  to  the  point  of  absurdity. 

Romanes  who  has  treated  the  evolution  of  instinct  at 
length  in  his  Mental  Evolution  in  Animals  regards  both 
natural  selection  and  use  inheritance  as  important  factors. 
Instincts  due  to  the  first  factor  he  calls  primary,  while 
those  due  to  the  latter  are  called  secondary.  Several  in- 
stances are  cited  in  which  it  is  claimed  there  is  strong  proof 
that  certain  instincts  have  been  produced  through  the  ac- 
cumulated effects  of  experience.     Other  instances  are  given 


THE  EVOLUTION  OF  INSTINCT  121 

which  point  to  a  blended  origin  of  instinct.  The  cases 
adduced  by  Romanes  have  been  critically  analyzed  by 
Lloyd  Morgan  and  Whitman,  who  have  shown  that  they  are 
of  very  doubtful  value  as  evidence,  and  we  need  not  repeat 
the  arguments  of  these  writers.  One  class  of  cases  adduced 
by  Romanes  we  shall  mention  since  it  shows  how  easily  the 
facts  may  be  misinterpreted.  Wildness  and  tameness  among 
birds  are  apparently  inherited  instincts.  Most  readers  are 
familiar  with  the  statement  that  birds  on  little  frequented 
islands  and  in  recently  explored  regions  betray  at  first  no 
marked  fear  of  man  and  may  frequently  be  knocked  over 
with  clubs,  whereas  in  places  where  they  have  been  hunted 
for  several  generations  they  become  very  wild,  the  fear  of 
man  being  shown  by  the  young  birds  as  well  as  the  old. 
This  fact  is  supposed  to  be  explainable  only  on  the  assump- 
tion that  the  painful  experiences  inflicted  by  man  have 
become  associated  with  the  appearance  of  the  human  form, 
and  that  this  association  is  transmitted  to  the  young  birds, 
giving  them  an  innate  fear  of  man  before  they  have  had  any 
experience  with  their  persecutor.  Those  who  have  adopted 
this  explanation  have  failed  to  consider  the  important  r6le 
of  imitation  in  the  behavior  of  young  birds.  Fear  is  un- 
doubtedly instinctive,  and  it  may  be  aroused  by  large  moving 
objects  or  unfamiliar  appearances  of  any  kindj  but  in  general 
it  may  be  said  that  a  sort  of  tradition  determines,  to  a  very 
large  extent,  the  objects  by  which  fear  is  awakened.  The 
excellent  observations  of  Hudson  on  fear  in  birds  have  con- 
vinced him  that  the  "fear  of  particular  enemies  is  in  nearly 
all  cases — ^for  I  will  not  say  all — ^the  result  of  experience  and 
tradition."  Young  birds  have  a  marked  proclivity  to  flee 
from  objects  at  which  their  parents  take  alarm^  and  to  scuiTy 
away  upon  hearing  the  parental  danger  signal.  Habits  of 
fear  in  regard  to  particular  enemies  are  rapidly  acquired  and 


122  THE  EVOLUTION  OF  INSTINCT 

passed  on.  Hudson  gives  among  other  illustrations  of  this 
fact  an  account  of  some  English  sparrows  which  he  was 
accustomed  to  feed  from  a  window.  "The  bread  and  seed 
were  thrown  on  to  a  low  roof  just  outside  the  window,  and  I 
noticed  that  the  young  birds  when  first  able  to  fly  were  al- 
ways brought  by  the  parents  to  this  feeding  place,  and  that 
after  two  or  three  visits  they  would  begin  to  come  of  their 
own  accord.  At  such  times  they  would  venture  quite  close 
to  me,  showing  as  little  suspicion  as  young  chickens.  The 
adults,  however,  although  much  less  shy  than  birds  of  other 
species,  were  extremely  suspicious,  snatching  up  the  bread 
and  flying  away;  or,  if  they  remained,  hopping  about  in  a 
startled  manner,  craning  their  necks  to  view  me,  and  mak- 
ing so  many  gestures  and  motions,  and  little  chirps  of  alarm, 
that  presently  the  young  would  become  infected  with  fear. 
The  lesson  was  taught  them  in  a  surprisingly  short  time; 
then  suspicion  was  seen  to  increase  from  day  to  day,  and 
about  a  week  later  they  were  scarcely  to  be  distinguished 
in  behavior  from  the  adults.  It  is  plain  that,  with  these 
little  birds,  fear  of  man  is  an  associate  feeling,  and  that, 
unless  it  had  been  taught  them,  his  presence  would  trouble 
them  as  little  as  does  that  of  a  horse,  sheep,  or  cow.'^ 

The  large  Rhea  of  South  America  which  is  used  for  food 
and  must  have  been  hunted  by  savages  for  a  very  long  period 
should  certainly  show  a  strong  innate  fear  of  man,  but  Hud- 
son found  that  the  young  captured  just  after  hatching  would 
follow  him  about  in  perfect  confidence.  WTien  he  would 
imitate  the  danger  note  of  the  parents  the  young  would  rush 
to  him  in  great  terror,  although  no  animal  was  in  sight. 
"If,"  says  Hudson,  "I  had  caused  a  person  to  dress  in  white 
or  yellow  clothes  for  several  consecutive  days,  and  had  he 
shown  himself  to  the  birds,  I  have  no  doubt  that  they 
would  soon  have  acquired  a  habit  of  running  in  terror  from 


THE  EVOLUTION  OF  INSTINCT  123 

him,  even  without  the  warning  cry,  and  that  the  fear  of  a 
person  in  white  or  yellow  would  have  continued  all  their 
lives." 

Birds  have  been  pursued  by  hawks  for  countless  gener- 
ations, but  there  is,  according  to  Lloyd  Morgan,  no  evidence 
that  they  have  an  instinctive  fear  of  hawks,  any  more  than 
of  large  moving  objects  in  general  which  are  seen  in  the  air. 
Such  fear  is  readily  taught  the  young,  and  different  species  of 
hawks,  as  Hudson  has  shown,  come  to  inspire  different 
degrees  of  fear  according  to  their  varied  powers  of  harm. 

While  instinctive  fear  of  particular  enemies  may  exist  in 
certain  animals,  the  evidence  that  it  has  in  any  case  been 
recently  acquired  is  entirely  inadequate.  There  seems  to 
be  more  evidence  that  wildness  has  been  partially  lost  in 
the  young  of  some  domesticated  animals.  Darwin  states 
that  "hardly  any  animal  is  more  difficult  to  tame  than  the 
young  of  the  wild  rabbit;  scarcely  any  animal  is  tamer  than 
the  young  of  the  domestic  rabbit  ^';  and  also  that  the  young 
of  the  tame  duck  are  more  tame  than  those  of  the  wild  duck 
— a  statement  which  is  confirmed  by  the  observations  of 
Dr.  Rae.  The  evidence  that  the  diminution  of  wildness  is 
due  to  the  disuse  of  the  instinct  is  not,  however,  sufficient. 
The  differences  may  have  been  due,  in  part  at  least,  to  dif- 
ferences among  the  wild  ancestry  of  the  species,  or  they  may 
have  arisen  through  selection,  perhaps  unconsciously,during 
domestication.  The  wilder  individuals  would  be  more  apt 
to  escape  or  fare  ill  than  the  tamer  ones,  and  there  would 
therefore  be  a  certain  tendency  for  selection  to  diminish  the 
wild  instinct.  It  would  require  more  careful  study  and 
comparison  of  the  instincts  of  the  young  of  domesticated 
and  of  wild  species  than  has  yet  been  made  to  furnish  ade- 
quate evidence  for  the  loss  of  fear  through  disuse. 

We  shall  not  enter,  at  any  great  length,  into  a  discussion  of 


124  THE  EVOLUTION  OF  INSTINCT 

the  supposed  influence  of  the  transmission  of  acquired 
characters  in  the  evolution  of  instinct.  The  fundamental 
question  is,  of  course,  a  biological  one,  and  in  whatever  way 
it  is  decided  the  psychologist  will  have  to  shape  his  theories 
accordingly.  Opinion  among  biologists  has  been  setting 
rather  strongly  against  neo-Lamarckism,  and  the  same 
tendency  is  evinced  among  many  writers  on  animal  beha- 
vior, such  as  Lloyd  Morgan,  Forel,  Groos,  Whitman,  Bald- 
win, and  others.  The  strong  resemblance  between  habits 
and  instincts  which  has  so  often  been  commented  upon, 
naturally  disposes  one  to  regard  the  latter  as  in  some  way 
derived  from  the  former;  nevertheless,  it  is  especially  in  the 
field  of  instinct  that  the  Lamarckian  theory,  which  at  first 
seems  so  plausible,  is  found  upon  critical  examination  to 
reveal  its  inadequacy. 

In  the  insects,  where  we  find  so  many  striking  examples 
of  almost  pure  instinct,  there  are  numerous  highly  complex 
instinctive  acts  which  are  performed  only  once  in  the  life- 
time of  the  individual.  The  larva  of  the  promethus  moth, 
for  instance,  when  nearly  ready  to  pupate,  spins  an  elabo- 
rate cocoon  in  which  it  passes  the  winter.  It  lines  the 
cocoon  with  a  loose  mass  of  silken  threads  which  will  lie 
next  to  its  body;  the  outer  layer  is  a  firm  resistant  coat  which 
is  admirably  adapted  to  keep  out  cold  and  moisture,  and  at 
one  end,  with  apparent  foresight,  there  is  left  an  opening 
filled  with  loose  silk  through  which  the  moth  may  push  its 
way  when  emerging  from  the  pupal  case.  Still  more  re- 
markable provision  is  apparently  manifested  in  the  way  in 
which  the  cocoon  is  attached;  it  is  usually  spun  against 
a  leaf  by  which  it  is  partly  enclosed,  and  to  guard  against 
falling  when  the  leaf  breaks  off,  a  strand  of  web  is  spun 
along  the  petiole  to  the  twig.  How  could  such  a  cocoon 
spinning  instinct  have  arisen?    Was  it  by  reflection  upon 


THE  EVOLUTION  OF  INSTINCT  125 

the  results  of  experiment?  To  anyone  who  has  made  any- 
first  hand  study  of  lepidopteran  psychology  the  supposition 
that  the  ingenious  mechanical  devices  shown  in  the  cocoon 
were  hit  upon  as  the  result  of  a  series  of  experiments  is 
about  as  probable  as  that  a  cat  would  be  able  to  comprehend 
the  differential  calculus.  Yet  we  read  such  phrases  as  "such 
calculation,"  "what  wonderful  contrivance,"  applied  to  such 
performances  by  some  of  the  foremost  students  of  animal 
psychology  a  couple  of  decades  ago.  How  absurd  they  all 
seem  now !  To  be  able  to  improve  a  habit  there  must  be  an 
opportunity  for  repeating  the  action.  CaterpOlars  might 
be  supposed,  by  the  mere  act  of  once  constructing  a  cocoon 
to  have  adapted  their  organization  to  this  operation,  and 
we  might  suppose  that  this  modification  affects  the  germ 
cells  so  that  the  next  generation  spins  with  somewhat 
greater  facility.  But  caterpillars  which  constructed  their 
cocoons  badly  would  have  no  opportunity  to  improve,  and 
their  bad  methods  would  be  handed  on,  and  confirmed 
more  and  more  in  their  badness. 

After  the  moth  emerges  from  the  cocoon  she  soon  deposits 
her  eggs  upon  a  species  of  plant  which  affords  suitable  food 
for  the  larvae.  How  did  the  moth  come  to  have  this  instinct? 
Can  any  one  believe  that  the  moth  watched  the  results  of 
laying  eggs  on  different  kinds  of  plants  and  formed  the 
habit  of  ovipositing  on  those  upon  which  the  larvae  happened 
to  thrive? 

Many  of  the  most  complex  instincts  of  insects  are  in  re- 
lation to  constructing  some  sort  of  protective  dwelling  for 
the  winter  and  in  making  provision  for  their  progeny,  and 
in  neither  of  these  cases  is  there,  in  most  forms,  room  for  im- 
provement through  profiting  by  failures.  Protective  dwel- 
lings are  usually  made  but  once,  and  in  providing  for  the 
young  there  is  usually  no  opportunity  for  the  parent  to 


126  THE  EVOLUTION  OF  INSTINCT 

observe  the  effect  of  its  operations,  so  that  if  it  makes 
mistakes  it  will  go  on  doing  so  indefinitely.  What  we  know  of 
insect  psychology  renders  it  entirely  out  of  the  question  to 
suppose  that  insects  would  be  able  to  reflect  upon  their  errors 
and  mend  their  ways  had  they  every  opportunity  to  observe 
their  deleterious  effects.  The  offspring,  instead  of  the  par- 
ents, have  to  take  all  the  consequences  of  the  mistakes,  and 
they  if  they  survived  would  certainly  not  have  enough  wit, 
if  they  had  the  desire,  to  make  things  any  better  for  the 
next  generation. 

As  was  first  pointed  out  by  Darwin,  an  important  objec- 
tion to  the  Lamarckian  theory  of  instinct  is  afforded  by  the 
instincts  of  worker  bees  and  ants.  The  instincts  of  these 
workers  are  among  the  most  wonderful  in  the  whole  animal 
kingdom,  and  they  are  much  more  varied  and  highly  devel- 
oped than  in  the  males  and  fertile  females.  As  Darwin 
remarks  **  peculiar  habits  confined  to  the  workers  or  sterile 
females,  however  long  they  might  be  followed,  could  not 
possibly  affect  the  males  and  fertile  females,  which  alone 
can  have  descendants.  I  am  surprised  that  no  one  has 
hitherto  advanced  this  demonstrative  case  of  neuter  insects 
against  the  well-known  doctrine  of  inherited  habit  advanced 
by  Lamark." 

In  the  hive  bee  the  activities  of  the  queen,  after  the  nuptial 
flight,  are  almost  entirely  confined  to  laying  eggs;  she  takes 
no  part  in  the  household  duties  of  the  hive  or  in  the  care  of 
offspring.  The  drone's  sole  function  in  life  is  to  impregnate 
the  queen;  he  takes  no  part  in  the  work  of  the  community. 
Gathering  honey,  making  comb,  caring  for  the  young, 
keeping  the  hive  clean,  etc.,  are  the  result  of  instincts  in 
the  worker  of  which  neither  of  the  parents  shows  the  least 
trace. 

This  apparently   crucial   argument  against  Lamarckism 


THE  EVOLUTION  OF  INSTINCT  127 

has  been  emphasized  by  Weismann  and  others,  but,  as 
Spencer  has  ably  shown,  the  case  is  not  so  easily  disposed  of. 
According  to  Spencer  the  fact  that  certain  structures  and 
instincts  occur  in  the  workers  which  are  not  found  in  the 
fertile  insects  is  not  because  the  workers  have  acquired  them 
since  their  separation  as  a  distinct  caste,  but  because  the 
fertile  insects  have  lost  them.  The  worker  caste  is  produced 
by  lack  of  sufficient  nutrition.  There  is  a  stunting  of  growth, 
a  failure  on  the  part  of  the  reproductive  organs  to  develop, 
and  in  ants  an  atrophy  of  the  wings  and  wing  muscles.  In 
the  early  stages  of  the  evolution  of  the  social  state  the  fertile 
female  possessed  all  the  instincts  for  making  the  nest, 
gathering  food  and  caring  for  the  young,  as  is  now  done  in  the 
more  primitive  social  conmiunities  of  bees  and  wasps.  In  so- 
cial wasps,  as  a  rule,  the  occupants  of  cells  richly  supplied  with 
food  emerge  as  fully  developed  females;  where  the  food  supply 
is  limited  the  females  are  smaller  and  sterile,  and  with  varied 
amounts  of  food  various  intermediate  gradations  of  size  are 
produced  from  the  largest  females  to  the  smallest  workers.  The 
smaller  individuals  which  on  account  of  the  partial  atrophy 
of  their  reproductive  systems  take  little  or  no  part  in  repro- 
duction busy  themselves  with  building  cells  and  storing  them 
with  food,  and  in  taking  care  of  the  young.  The  fertile 
females  live  with  the  rest  of  the  community  and  share  its 
labors,  but  in  the  following  year  they  scatter  and  form  the 
nuclei  of  new  colonies.  At  first  the  queen  starts  the  nest, 
rears  and  cares  for  the  young,  and  performs  all  the  tasks  in- 
cidental to  her  small  household  economy.  When  the  young 
emerge  they  cooperate  in  the  work  of  the  queen.  As  the 
conmiunity  grows  the  queen  gradually  \sdthdraws  herself 
from  the  labors  shared  by  the  workers  and  devotes  herself 
more  and  more  to  laying  eggs. 
Among  bees  there  are  numerous  gradations  between  the 


128  THE  EVOLUTION  OF  INSTINCT 

solitary  forms  and  the  highly  organized  social  state  repre- 
sented by  the  hive  bee.  The  various  grades  of  social  life 
which  these  insects  present  have  been  described  in  a  very 
interesting  little  treatise  by  Buttel-Reepen  on  '^Die  stam- 
msgeschichtliche  Entstehung  des  Bienenstaates.'^  The  series 
of  forms  which  Buttel-Reepen  describes  indicates  very 
clearly  that  there  has  been  a  gradual  specialization  of 
function  in  the  queen  from  a  condition  in  which  she  performed 
all  the  duties  of  the  household  to  one  in  which  her  functions 
are  confined  practically  to  reproduction.  A  sort  of  division 
of  labor  has  come  about  so  that  the  workers  and  the  queen 
together  perform  the  labors  formerly  accomplished  by 
the  queen  alone.  Herbert  Spencer  is  therefore  in  the  main 
right  in  his  contention  that  the  fact  that  the  neuters  have 
instincts  not  found  in  the  queen  is  due  to  the  queen^s  having 
lost  them  in  specializing  in  the  direction  of  increased  capacity 
for  reproduction.  It  is  possible  to  maintain,  therefore,  that 
the  transmission  of  acquired  characters  may  have  moulded 
the  instincts  of  the  worker  caste  up  to  that  point  in  social 
evolution  at  which  they  came  to  be  practically  sterile. 

The  differences  between  the  queen  and  worker  are 
occasioned  by  differences  in  food.  With  a  diminution  of 
the  food  supply  and  a  consequent  arrest  of  development 
of  the  organs  of  reproduction  there  is  a  suppression  of  the 
reproductive  instincts,  but  the  fostering  and  household 
instincts  are  retained.  The  worker,  we  might  say,  halts  at 
a  phyletically  older  period.  The  queen  with  a  richer  food 
supply  develops  beyond  to  a  stage  in  which  what  might 
have  become  worker  characters  are  suppressed  in  the  interest, 
perhaps  of  greater  reproductive  efficiency.  The  condition 
might  be  compared  to  what  is  found  in  certain  parasitic 
isopods  in  which  the  adult  female  after  having  passed  through 
active  and  highly  organized  stages  of  metamorphosis  comes 


THE  EVOLUTION  OF  INSTINCT  129 

to  degenerate  into  an  almost  shapeless  mass  devoted  entirely 
to  feeding  and  the  production  of  eggs.  If  now  the  develop- 
ment of  some  of  the  females  could  be  checked,  by  insuffi- 
cient nutrition  at  an  earlier  period  of  metamorphosis  we 
should  have  a  caste  of  sterile  forms  more  highly  organized  in 
structure  and  of  more  varied  instincts  than  the  fertile  females. 
It  would  certainly  be  an  error  to  conclude  that  the  superiority 
of  the  sterile  individuals  was  due  to  their  having  independ- 
ently acquired,  since  the  differentiation  of  the  caste,  charac- 
ters which  were  not  a  part  of  the  legacy  of  their  parents. 
This  is,  I  think,  a  fair  statement  of  the  neo-Lamarckian 
side  of  the  question.  Granting  that  the  theory  is  otherwise 
acceptable  the  Lamarckian  doctrine  is  still  open  to  serious  ob- 
jections when  applied  to  the  instincts  of  neuter  insects. 
It  would  compel  us  to  assume  that  all  the  adaptive  struc- 
tures and  instincts  of  the  worker  bee  were  but  the  survival  of 
a  more  primitive  period  of  social  evolution  before  the  complete 
separation  of  the  fertile  and  sterile  castes,  when  presumably 
they  might  have  been  evolved  through  the  inherited  effects 
of  habit.  And  not  only  should  we  have  to  assume  that  there 
had  been  no  improvement  in  the  non-reproductive  activities 
since  the  queen  had  ceased  performing  them,  but  the  theory 
would  lead  us  to  expect  a  certain  amount  of  deterioration 
through  disuse.  It  certainly  cannot  be  supposed  that 
during  a  period  long  enough  for  the  queen  to  lose  her  wax 
glands,  pollen  basket,  and  all  her  instincts  for  gathering 
honey,  making  comb,  and  caring  for  the  young,  the  structures 
and  instincts  of  the  workers  should  have  suffered  no  degenera- 
tion. Instead  of  this  there  has  undoubtedly  been  a  decided 
improvement.  Nowhere  in  social  bees  do  we  find  so  high  a 
degree  of  structural  adaptation,  such  varied  activity,  such 
perfect  workmanship  in  the  construction  of  comb  with  its 
varied  cells  for  queen,  drone  and  worker,  and  such  complex 


130  THE  EVOLUTION  OF  INSTINCT 

social  life  as  among  the  workers  of  the  hive  bee.  If  so  con- 
siderable a  portion  of  this  remarkable  evolution  has  occurred 
since  the  separation  of  castes  excluded  the  Lamarckian  factor 
from  playing  any  helpful  part,  we  might  conclude  that  the 
Lamarckian  theory  is  not  necessary  in  order  to  account  for 
the  earlier  stages  of  social  progress. 

In  the  hive  bee  we  not  only  have  the  instincts  of  the  more 
primitive  forms  carried  to  a  high  degree  of  perfection  and 
specialization,  but  we  have  new  instincts  which  must  have 
arisen  since  the  separation  of  the  worker  caste,  since  they 
have  a  direct  relation  to  this  very  separation.  Such,  for 
instance,  are  shown  in  the  remarkable  behavior  of  bees 
when  deprived  of  their  queen.  When  the  queen  is  removed 
from  the  hive  and  there  is  no  other  soon  to  hatch,  the  workers 
destroy  a  number  of  old  cells  and  construct  a  new  queen 
cell,  and  then  proceed  to  feed  the  young  grub  that  is  en- 
closed on  liberal  quantities  of  royal  jelly  by  means  of  which 
it  is  caused  to  develop  into  a  new  queen.  In  this  case  of 
the  instincts  which  lead  the  bees  to  regulate  the  supply 
of  queens  the  theory  of  inherited  effects  of  experience, 
as  Spencer  has  advocated  it,  cannot  apply.  The  instinct 
cannot  have  antedated  the  separation  of  the  castes  because 
it  is  based  upon  the  existence  of  caste  differences. 

Other  difficulties  are  presented  by  the  differentiations 
within  the  neuter  class  among  ants  and  termites  where  there 
occur  two  or  more  kinds  of  sterile  insects  adapted  by  struc- 
ture and  instinctive  endowment  for  different  functions  in 
the  community.  In  many  cases  the  evidence  clearly 
points  to  the  evolution  of  new  adaptive  characters  in  the 
worker  caste,  instead  of  merely  the  degeneration  of  the 
fertile  insects.  And  where  this  has  occurred  the  Lamarckian 
theory  fails  to  explain  the  facts. 

In  the  chapter  on  Instinct  in  the  Origin  of  Species  Darwin 


THE  EVOLUTION  OF  INSTINCT  131 

attempted  to  show  that  the  main  factor  to  which  instincts 
owe  their  origin  is  natural  selection.  The  necessity  for  an 
appeal  to  a  previous  intelligence  was  swept  away,  and  while 
Darwin  did  not  deny  that  the  inheritance  of  acquired  char- 
acters played  a  part  in  the  development  of  instinct,  he 
ascribed  to  this  factor  a  very  subordinate  rdle,  and,  as  we 
have  seen,  pointed  out  some  very  serious  objections  which 
beset  the  theory. 

Darwin's  theory  assumes  that  instincts  vary.  Animals 
which  are  endowed  with  congenital  variations  of  instinct 
which  are  advantageous  to  them  will,  other  things  equal, 
survive;  those  which  have  injurious  variations  will  tend  to 
perish.  Fortunate  variations  of  behavior  may  thus  be 
accumulated  along  useful  lines  and  build  up  complex  and 
highly  adaptive  modes  of  behavior.  That,  as  the  theory 
requires,  instincts,  like  corporeal  structm-es,  are  subject  to 
congenital  variations  we  have  abundant  evidence.  We  should 
of  course  expect  that  variations  in  instinct  would  follow 
variations  in  structure,  but  we  should  scarcely  expect  from 
this  standpoint  to  find  instincts  so  variable  as  they  are. 
Apparently  a  slight  structural  variation  may  produce  a 
variation  in  instinctive  behavior  that  seems  out  of  all  pro- 
portion to  the  cause. 

Some  of  the  most  careful  investigations  of  variations  of 
instinct  have  been  made  by  the  Peckhams  in  their  classical 
studies  on  the  instincts  of  solitary  wasps.  Concerning 
Anmiophila  which  stores  its  nest  with  caterpillars  which 
it  paralyzes  by  stinging  them  in  the  ventral  ganglia,  the 
Peckhams  remark:  "In  the  three  captures  that  came 
under  our  observation,  all  the  caterpillars  being  of  the  same 
species  and  almost  exactly  of  the  same  size,  three  different 
methods  were  employed.  In  the  first  seven  stings  were 
given  at  the  extremities,  the  middle  segments  being  left 


132  THE  EVOLUTION  OF  INSTINCT 

untouched,  and  no  malaxation  was  practised.  In  the 
second  seven  stings  again,  but  given  in  the  anterior  and 
middle  segments,  followed  by  slight  malaxation.  In  the 
third  only  one  sting  was  given,  but  the  malaxation  was 
prolonged  and  severe/'  The  severity  of  the  stinging  as 
indicated  by  the  conditions  of  the  caterpillars  stored  in  the 
nest  also  varies.  Of  fifteen  caterpillars  which  were  stung 
by  Ammophila  umaria  the  Peckhams  found  that  ''  some  of 
them  lived  only  three  days,  others  a  little  longer,  while 
still  others  showed  signs  of  life  at  the  end  of  two  weeks.'' 
And  in  summing  up  their  observations  on  the  stinging  in- 
stinct these  writers  state  that  "out  of  forty-five  species  of 
our  solitary  wasps  about  one-third  kill  their  prey  outright. 
Of  those  that  remain  there  is  not  a  single  species  in  which 
the  sting  is  given  with  invariable  accuracy.  To  judge  from 
results  they  scarcely  sting  twice  alike,  since  the  victims  of 
the  same  wasp  may  be  killed  at  once  or  may  live  from  one 
day  to  six  weeks,  or  perhaps  ultimately  recover.  Even  the 
caterpillars  of  Ammophila,  the  most  distinguished  surgeon, 
live  anywhere  from  two  to  forty  days." 

Contrary  to  the  conclusions  of  Fabre,  who  contended  that 
the  instincts  of  Ammophila  are  practically  undeviating,  the 
Peckhams  remark  that  "the  one  preeminent,  unmistakable 
and  ever  present  fact  is  variability.  Variability  in  every 
particular — ^in  the  shape  of  the  nest  and  the  manner  of  dig- 
ging it,  in  the  condition  of  the  nest  (whether  closed  or  open) 
when  left  temporarily,  in  the  method  of  stinging  the  prey, 
in  the  degree  of  malaxation,  in  the  manner  of  carrying  the 
victim,  in  the  way  of  closing  the  nest,  and  last,  and  most 
important  of  all,  in  the  condition  produced  in  the  victims  by 
stinging." 

Among  certain  species  of  ants,  Polyergus  rufescens  and 
several  species  of  Formica  and  Lasius,  whose  larvse  ordinarily 


THE  EVOLUTION  OF  INSTINCT  133 

spin  a  cocoon,  there  are  frequently  individuals  which  fail  to 
spin,  and  apparently  they  suffer  no  injury  from  this  lack  of 
covering.  It  is  said,  by  many  writers,  that  there  is  great 
variation  in  the  pugnacity  of  ants,  some  individuals  attacking 
all  sorts  of  enemies  with  the  greatest  ferocity,  while  others 
are  so  cowardly  that  they  flee  on  the  least  intimation  of 
danger.  In  honey  bees  some  forms  have  the  instinct  of 
making  drone  cells  greatly  exaggerated  and  there  are  great 
variations  in  the  pugnacity  of  different  stocks. 

Careful  studies  which  have  been  made  of  the  behavior  of 
the  crayfish,  the  earthworm,  Hydra  and  various  protozoa 
have  revealed  a  surprising  degree  of  individual  variability. 
In  the  field  of  tropisms  striking  variations  are  frequent 
among  animals  subjected  to  the  same  external  conditions. 
Passing  to  higher  organisms  we  find  numerous  records  of 
the  variation  of  instinct,  especially  in  birds  and  mammals. 
Variations  in  the  nest  building  of  birds  are  common,  and 
numerous  instances  have  been  compiled  by  Darwin  in  his 
posthumous  essay  on  instinct.  Beclistein  states  that  in  the 
nightingale  some  individuals  show  an  inherited  tendency 
to  sing  in  the  daytime  instead  of  at  night.  There  are  many 
instances  in  which  birds  have  apparently  lost  the  instinct 
of  migration.  Certain  breeds  of  domestic  fowl  have  lost  the 
instinct  to  incubate  their  eggs  and  among  other  breeds  this 
instinct  is  notoriously  variable. 

Trainers  of  animals  frequently  remark  upon  the  striking 
differences  which  are  presented  not  only  in  aptitude  for  learn- 
ing but  in  the  habits  and  disposition  of  different  individuals. 
Yerkes  finds  in  the  dancing  mouse  marked  differences  in 
sensitiveness  to  visual,  auditory,  tactual  and  olfactory 
stimuli  in  individuals  of  the  same  age  and  sex;  striking  differ- 
ences also  occur  in  their  general  habits  and  in  docility. 
The  peculiar  dancing  proclivity  of  this  variety  is  of  uncertain 


134  THE  EVOLUTION  OF  INSTINCT 

origin,  but  it  is  a  suggestive  fact  that  the  same  trait  in  vary- 
ing degrees  occasionally  crops  out  in  other  varieties  of  mice, 
and  Haake  has  described  a  similar  curious  variation  in  a 
species  of  shrew.  Hereditary  peculiarities  of  movement 
have  been  described  many  times  both  in  man  and  in  animals. 
Darwin  quotes  an  account  communicated  by  the  Rev.  W. 
Darwin  Fox  of  a  terrier  which  when  begging  moved  her 
paws  in  a  very  peculiar  manner  very  different  from  that  of 
other  dogs;  "her  puppy,  which  never  could  have  seen  her 
mother  beg,  now  when  full  grown  performs  the  same  peculiar 
movement  exactly  in  the  same  way.  Another  peculiar  he- 
reditary variation  is  reported  in  a  letter  by  Dr.  Huggins  to 
Darwin,  of  an  English  mastiff  which  when  first  taken  out, 
at  the  age  of  six  weeks,  from  the  house  where  he  was  born 
started  back  in  alarm  at  the  first  butcher  shop  he  had  seen. 
Later  when  the  endeavor  was  made  to  get  him  past  the 
butcher  shop  he  threw  himself  down  and  could  not  be  induced 
by  coaxing  or  threats  to  pass  the  shop.  On  enquiry  it 
was  found  that  the  same  peculiai-  antipathy  was  possessed  by 
the  father  of  the  dog,  by  the  grandfather  and  by  two  others 
of  the  latter's  descendants 

Illustrations  of  variations  in  instinctive  behavior  might 
be  multiplied  almost  indefinitely,  but  what  has  been  said  will 
perhaps  suffice  to  give  some  indication  of  the  prevalence 
of  such  variation  throughout  the  animal  kingdom.  So  far 
as  can  be  determined,  variations  of  instinct  have  little  regard 
to  utility;  they  may  be  of  service  to  their  possessors,  or, 
like  Huggins'  case  of  the  inborn  aversion  of  a  dog  to  butcher 
shops,  of  no  particular  value,  or  positively  injurious  as  in 
the  occasional  deterioration  of  the  instinct  of  incubation. 
As  instincts  are  no  less  important  than  corporeal  structures 
in  the  struggle  for  existence,  we  can  readily  conceive  how 
useful  variations  may  be  accumulated  by  natural  selection, 


THE  EVOLUTION  OF  INSTINCT  135 

and  in  the  case  of  many  instincts  it  is  not  difficult  to  picture 
to  one's  self  how  natural  selection  may  have  brought  them  to 
their  present  state  of  perfection.  To  find,  as  we  can  in  many 
cases,  instincts  in  various  grades  of  development  in  alhed 
species  of  animals,  while  it  shows  the  general  course  of  their 
evolution,  tells  us  little  of  their  method  of  evolution.  That 
instincts  are  variable,  that  they  are  often  imperfect,  that  their 
general  course  of  development  has  been  along  adaptive  lines, 
and  that,  as  Darwin  emphasized,  they  are  always  primarily 
of  value  to  the  species  possessing  them  and  only  incidentally 
of  service  to  others,  are  facts  indicative  of  the  potency 
of  natural  selection  in  the  evolution  of  instinctive  be- 
havior. 

Evidence  for  the  potency  of  selection  is  furnished  by  the 
study  of  the  striking  modifications  of  instinct  which  have 
taken  place  in  animals  under  domestication.  While  only 
rarely  has  the  attempt  been  made  to  modify  instincts  along 
particular  lines  by  selection,  yet  they  have  doubtless  been 
modified  as  the  incidental  result  of  selection  on  other  lines. 
A  sort  of  unconscious  selection  has  probably  played  a  part 
in  modifying  especially  the  emotional  characteristics  of 
animals.  Ugly  and  vicious  dispositions  in  dogs,  for  instance, 
would  tend  to  be  eliminated,  and  the  qualities  of  affection, 
fidelity  and  other  traits  which  commend  the  animals  to  the 
good  graces  of  their  keepers  would  be  fostered.  The 
useful  instincts  of  the  pointer  and  the  setter,  however  they 
may  have  made  their  beginning,  have  certainly  been  devel- 
oped to  a  considerable  degree  by  continued  selection.  The 
curious  instincts  of  tumbling  and  pouting  which,  as  Wliitman 
has  shown,  have  their  basis  in  traits  of  behavior  found  in 
pigeons  in  general,  have  been  developed  by  the  efforts  of 
fanciers  to  an  almost  monstrous  degree.  The  Indian  sub- 
breed  of  tumblers  which  has  been  bred  for  at  least  two-hundred 


136  THE  EVOLUTION  OF  INSTINCT 

and  fifty  years  tumbles  even  when  on  the  ground  and  con- 
tinues to  do  so  until  taken  up. 

Whether  or  not  other  factors  have  piayed  a  part  in  the 
evolution  of  behavior  we  shall  not  here  discuss.  Certainly 
their  claims  cannot  be  said,  at  present,  to  rest  upon  a  very 
satisfactory  basis.  If  it  may  savor  of  dogmatism  to  contend 
for  the  all-sufficiency  of  natural  selection,  it  is  but  an  ex- 
hibition of  folly  to  reject  as  of  little  worth  the  only  hypothe- 
sis by  which  we  can  account  for  much  of  the  evolution  along 
adaptive  lines  which  has  taken  place. 

If  we  reject  tho  Lamarckian  theory  it  is  still  possible  to 
conceive  how  the  activities  of  organisms  may  after  all  have 
a  guiding  influence  upon  the  course  of  evolution.  It  is  a 
somewhat  striking  coincidence  that  three  writers,  J.  Mark 
Baldwin,  H.  F.  Osborn,  and  C.  Lloyd  Morgan,  put  forward 
independently,  and  at  nearly  the  same  time,  a  theory  to 
explain  how  this  guiding  influence  might  take  place  with- 
out having  recourse  to  the  Lamarckian  factor.  There  is 
considerable  evidence  from  fossil  forms,  the  structural 
adaptations  of  living  organisms,  and  the  interrelations  of 
structure  and  behavior,  which  indicates  that  evolution  has 
proceeded  along  lines  corresponding  to  the  modifications 
produced  in  the  individual  by  its  own  activities.  There  can 
be  no  doubt  that  the  adaptations  acquired  through  these 
activities  have  frequently  enabled  the  organism  to  survive 
in  the  struggle  for  existence.  This  power  of  individual  ac- 
commodation, like  other  characteristics  of  the  organism,  is 
subject  to  a  certain  degree  of  congenital  variability.  It  fol- 
lows that  those  congenital  variations  which  enable  the  organ- 
ism to.  acquire  adaptive  modifications  with  greater  readiness 
will  be  preserved,  and  consequently  variations  in  the  direc- 
tion of  these  acquired  modifications  will  accumulate.  Were 
it  not  for  the  adaptations  acquired  through  the  organism's  ac- 


THE  EVOLUTION  OF  INSTINCT  137 

tivity  congenital  variations  in  certain  directions  might  never 
have  risen  to  the  point  at  which  they  would  be  of  service. 
Unless  supplemented  by  acquired  modifications  they  may 
never  have  attained  a  selective  value.  We  might  say  then 
that  it  is  the  activities  of  the  organism  which  to  a  consider- 
able degree  determine  the  selective  value  of  its  congenital 
variations.  As  Baldwin  remarks,  "Congenital  variations, 
on  the  one  hand,  are  kept  ahve  and  made  effective  by  their 
use  for  adjustments  in  the  life  of  the  individual;  and,  on  the 
other  hand,  adaptations  become  congenital  by  further  pro- 
gress and  refinement  of  variation  in  the  same  lines  of  function 
as  those  which  their  acquisition  by  the  individual  called 
into  play.  But  there  is  no  need  in  either  case  to  assume 
the  Lamarckian  factor.  In  cases  of  conscious  adaptation, 
we  reach  a  point  of  view  which  gives  to  organic  evolution  a 
sort  of  intelligent  du'ection  after  all;  for  of  all  the  variations 
tending  in  the  direction  of  an  adaptation,  but  inadequate 
to  its  complete  performance,  only  those  will  be  supplemented 
and  kept  alive  which  the  intelligence  ratifies  and  uses.'' 

It  is  in  the  evolution  of  instinct,  if  anywhere,  that  the  factor 
of  organic  selection  would  appear  to  be  especially  potent. 
The  practical  outcome  of  its  operation  is  much  the  same 
as  if  the  effects  of  experience  were  actually  inherited,  and  we 
are  thus  enabled  to  explain,  in  terms  of  selection,  phenomena 
which  at  first  appear  to  furnish  strong  evidence  for  the 
transmission  of  acquired  characters. 

While  we  may  recognize  the  value  of  the  theory  of  orgamc 
selection,  it  is  not  clear  that  we  should  regard  it  as  a  new 
factor  in  evolution.  It  cannot,  in  strictness,  be  regarded  as 
a  compromise  between  Neo-Darwinism  and  Neo-Lamarckism. 
Rather  it  is  one  of  the  ways  in  which  natiu*al  selection  may 
be  conceived  to  act.  It  consists  of  the  selection  of  those 
congenital  variations  which  facilitate  the  acquirement  of 


138        '       THE  EVOLUTION  OF  INSTINCT 

adaptations.  It  is  still  natural  selection;  but  the  theory- 
explains  how,  through  natural  selection,  acquired  modifica- 
tions may  determine  the  direction  of  evolution. 

BIBLIOGRAPHY 

Baldwin,  J.  M.     Development  and  Evolution.     N.  Y.,  '02. 

Ball,  W.  P.     Neuter  Insects  and  Lamarkism,  Nat.  Sci.,  4,  91,  '94. 

Cunningham,  J.  T.  Neuter  Insects  and  Darwinism,  Nat.  Sci.,  4, 
281,  '94. 

Darwin,  C.     Origin  of  Species,  '59. 

EiMER,  0.  E.     Organic  Evolution,  '90. 

Lewes,  G.  H.    Problems  of  Life  and  Mind,  London,  '74-'80. 

Morgan,  C.  L.  Animal  Life  and  Intelligence,  '91.  Habit  and 
Instinct,  London,  '96.     Animal  Behaviour,  London,  'GO. 

Romanes,  G.  J.     Mental  Evolution  in  Animals,  London,  '85. 

Spencer,  H.  Principles  of  Psychology,  1855.  A  rejoinder  to 
Professor  Weismann,  Contemporary  Rev.,  64,  893,  '93. 

Weismann,  a.  The  All-sufficiency  of  Natural  Selection.  Con- 
temporary Rev.,  '64,  309  and  596,  '93.    The  Evolution  Theory, 

2  vols.,  '04. 

Whitman,    C.    O.    Animal    Behavior,    Wood's    Hole    Biological 

Lectures  for  1898,  '99. 

Wundt,   W.     Lectures  on  Human  and  Animal  Psychology,   '94. 


CHAPTER  VII 

THE  NON-INTELLIGENT  MODIFICATIONS  OF 
BEHAVIOR 

"  Both  elements,  automatism  and  plasticity,  are  found  in  different 
proportions  with  all  animals  from  the  highest  to  the  lowest." — 
Wasmann,  Psychology  of  Ants  and  of  Higher  Animals. 

While  there  is  in  all  organisms  a  certain  measure  of  useless 
if  not  positively  injurious  activity,  the  organic  mechanism 
is  a  self  regulating  one,  and  meets  varied  conditions  of 
life  with  appropriate  changes  of  response.  These  adaptive 
changes  in  relation  to  different  circumstances  are  found  even 
in  the  lowest  organisms.  With  the  evolution  of  life  they  be- 
come more  varied  and  more  specialized  and  contribute  to  the 
development  of  intelligence  which  may  be  regarded  as  but 
one  species  of  the  comparatively  large  genus  of  adaptive 
variation  of  behavior.  In  accordance  with  a  common 
usage  the  term  intelligence  is  here  restricted  to  those  forms 
of  behavior  based  upon  the  formation  of  associations. 
As  Spencer  has  shown,  intelligence  is  a  part  of  the  general 
process  of  adjustment  which  makes  up  the  behavior  of  an 
animal.  To  a  certain  extent  behavior  is  stereotyped  and 
to  a  certain  extent  it  is  plastic.  Both  kinds  of  behavior  are 
necessary  in  varying  degrees  in  the  life  of  all  animals.  In 
this  chapter  we  shall  consider  some  of  the  plastic  features 
of  behavior  in  which  the  element  of  association  is  not  involved. 

ACTION  OF  COMBINED  STIMULI 

Reaction  to  any  given  stimulus  is  often  interfered  with  by 
a  tendency  to  react  to  other  stimuli  received  at  the  same 

139 


140  MODIFICATIONS  OF  BEHAVIOR 

time.  An  electric  current  which  would  cause  a  Paramoecium 
if  swimming  freely  in  the  water  to  turn  to  the  cathode,  will 
usually  produce  no  movement  in  a  specimen  in  contact 
with  a  solid.  The  reactions  of  Paramoecium  to  chemical, 
thermal  and  mechanical  stimuli  are  also  greatly  influenced 
by  the  same  factor.  Earthworms,  as  Darwin  has  observed, 
usually  fail  to  react  to  light  when  mating  or  feeding. 
A  striking  illustration  of  the  influence  of  the  substratum 
in  reactions  to  light  is  afforded  by  the  leech  Branchellion. 
When  in  a  dish  of  water  the  leech  reacts  to  a  passing  shadow 
by  raising  its  anterior  end  and  swaying  it  about.  If  the 
leech  is  in  contact  with  its  host,  the  torpedo,  it  pays  not  the 
slightest  heed  to  passing  shadows.  In  Ranatra  the  positive 
response  to  light  is  checked  when  the  animal  is  feeding  or 
cleaning  itself  and  is  quickly  resumed  when  these  operations 
are  completed.  And  in  fiddler  crabs  the  positive  reaction 
to  light  may  be  overcome  by  fear  of  an  approaching  object, 
although  with  longer  exposure  to  light  the  phototactic  re- 
sponse becomes  the  more  potent  one.  Contact  in  many  lower 
animals  inhibits,  as  we  have  seen  in  a  previous  chapter,  re- 
actions to  light  and  many  other  stimuli,  and  in  higher  forms 
it  may  profoundly  modify  behavior  in  relation  to  enemies. 
To  a  considerable  degree  these  changes  of  behavior  are  the 
results  of  the  simple  interference  of  reactions;  but  stimuli 
may  act  in  such  a  way  that  they  produce  a  marked  physio- 
logical change  in  the  organism,  as  when  a  contact  stimulus 
brings  about  the  death  feint,  and  the  lack  of  responsiveness 
is  due  then  to  the  induced  condition  rather  than  to  an  an- 
tagonistic movement. 

DIMINUTION  OF  REACTION  TO  REPEATED  STIMULATION 

Modifications  of  reaction  due  to  the  simultaneous  reception 
of  other  stimuli  are  closely  affiliated  with  modifications  due 


DIMINUTION  OF  REACTION  141 

to  previous  stimuli.  A  single  stimulus  when  repeated  with 
sufficient  frequency  sooner  or  later  brings  about  a  change 
of  response.  The  most  usual  modification  is  a  gradual 
cessation  of  the  reaction.  Light  mechanical  stimulation  of 
the  anterior  end  of  Loxophyllum  causes  at  first  a  ready 
response;  several  repetitions  in  close  succession  diminish 
the  responsiveness  so  that  a  much  stronger  stimulus  often 
fails  to  produce  any  noticeable  effect.  In  a  few  seconds, 
however,  recovery  is  apparently  complete  and  the  animal 
is  as  responsive  as  before.  In  Stentor,  as  we  have  seen, 
after  a  few  light  contact  stimuli  no  further  contraction  takes 
place.  Hydra,  according  to  Wagner,  if  subjected  to  a 
slight  mechanical  stimulus  such  as  is  caused  by  tapping  on 
the  object  on  which  it  rests,  usually  contracts  completely. 
"As  the  tapping  continues  this  state  of  contraction  is 
maintained  for  several  seconds,  sometimes  even  from  one- 
half  to  one  minute;  but  sooner  or  later,  in  spite  of  continuous 
stimulation,  the  Hydra  slowly  expands.  When  it  has  reached 
its  normal  state  of  expansion  it  remains  in  that  position 
as  long  as  the  stimulus  is  not  increased,  and  even  when  it 
is  slightly  increased.  ...  If  the  interval  between  the 
stimuli  is  considerably  increased  so  as  to  allow  the  Hydra 
to  expand  fully  after  each  contraction,  the  tap  being  given 
the  moment  expansion  ceases,  the  result  is  a  different  one. 
There  is  in  this  case  no  change  in  the  reaction  after  repeated 
stimulation.  .  .  .  Recovery  from  the  acclimatizing  effect 
must,  therefore,  be  very  rapid.  *' 

Walter  found  that  a  planarian,  if  lightly  jarred  during 
its  gliding  movements,  would  halt  momentarily  and  then 
continue  its  course.  If  the  stimulus  was  repeated  at  inter- 
vals of  a  second  the  worm  would  halt  with  less  and  less 
certainty  and  finally  glide  along  undisturbed.  The  effect 
of  the  stimulation  was  very  evanescent  for  after  a  minute 


142  MODIFICATIONS  OF  BEHAVIOR 

the  behavior  of  the  worm  was  the  same  as  before.  The 
writer  has  found  that  mosquito  larvae  which  descend  quickly 
from  the  surface  of  the  water  when  a  shadow  passes  over 
them,  cease  to  react  after  a  number  of  trials  but  become 
as  responsive  as  ever  after  a  short  rest. 

Many  forms  which  react  by  contraction  to  a  sudden  increase 
or  decrease  of  illumination  show  similar  modifications.  The 
leech  Clepsine,  when  a  shadow  is  cast  upon  it,  raises  and 
elongates  the  anterior  end  of  the  body,  but  after  a  number  of 
such  reactions  shadows  produce  no  result.  Mrs.  Yerkes  in 
experimenting  on  a-  tube  dwelling  annelid  Hydroides  found 
that  shadows  which  at  first  caused  the  worm  to  retract  into 
its  tube,  would,  if  repeated  at  brief  intervals,  produce  no 
response.  With  longer  intervals  the  response  was  more 
regular.  Hargitt  in  studying  the  same  form  arranged  a 
pendulum  so  that  it  would  throw  a  shadow  on  the  worm 
at  regular  intervals  and  found  that  "with  the  full  second 
movement  there  was  more  or  less  constant  reaction  with 
each  passing  shadow.  With  the  half  second  movement  it 
was  found  that,  after  the  first  few  beats,  a  considerable 
portion  of  the  worms  failed  to  respond  at  all,  and  with 
the  quarter  second  beats  almost  all  the  colonies  became 
indifferent  to  the  presence  of  the  passing  shadows."  A 
condition  which  Hargitt  considers  akin  to  fatigue  is  finally 
produced,  in  which  the  organism  becomes  comparatively 
irresponsive  to  light.  Walter  in  his  studies  on  the  reactions 
of  planarians  to  light  finds  that  "  when  worms  were  placed 
in  a  field  of  non-directive  light,  parts  of  which  were  of  two 
different  intensities,  the  number  of  wigwag  responses 
made  at  the  critical  line  separating  the  two  intensities 
grew  less  after  the  animals  had  repeatedly  crossed  the  line. 
At  first  the  new  condition  of  sharply  contrasted  light  in- 
tensities in  the  worm's  field  of  locomotion  called  out  a  large 


DIMINUTION  OF  REACTION  143 

percentage  of  wigwag  responses.  Later,  however,  by 
repeated  experiences,  the  worm  became  familiar  with  this 
feature  of  its  environment  and  made  fewer  wigwag  motions,'' 
the  percentage  in  a  specimen  of  Planaria  gonocephala  be- 
coming gradually  reduced  from  84  per  cent,  in  the  first 
twenty-five  crossings  to  32  per  cent,  in  the  fourth  twenty- 
five. 

There  are  many  bivalve  molluscs  w^hich  retract  their 
extended  siphons  or  close  their  shells  upon  being  stimulated 
by  shadows,  or  in  some  cases  when  subjected  to  increased 
illumination,  but  after  one  or  more  repetitions  of  the  stimulus, 
the  number  varying  greatly  in  different  species,  the  response 
is  no  longer  made.  Gradual  cessation  of  response  to  a  given 
stimulus  may  be  due  to  (1)  a  fatigue  of  the  motor  apparatus, 
(2)  to  a  dulling  of  the  sensibility  of  receptors,  or  (3)  perhaps 
to  other  states  of  the  organism.  With  the  first  of  these  is 
not  improbably  to  be  classed  the  gradual  diminution  of  the 
duration  of  death  feint,  which  maaiy  animals  show  in  succes- 
sive feints.  The  death  feigning  attitude  is  one  in  which 
the  muscles  are  usually  rigid  and  the  condition  naturally 
involves  more  or  less  fatigue.  As  the  feints  decrease  in 
length  the  body  also  becomes  more  relaxed  and  the  attitude 
of  the  body  less  constant  and  characteristic.  In  lower 
forms  the  death  feint  but  slowly  wears  itself  out.  In  higher 
ones  feigning  is  carried  out  only  once  or  a  very  few  times  be- 
fore the  animal  refuses  to  feign  longer.  Hence  in  many  cases 
the  factor  of  intelligence  may  come  into  play,  but  whether 
this  will  explain  all  cases  where  the  response  is  given  up  sud- 
denly appears  doubtful. 

A  considerable  part  of  the  cases  of  cessation  of  reaction  to  a 
given  stimulus  are  probably  due,  as  Bohn  contends,  to  a 
''fatigue  sensorielle''  or  dulling  of  sensibility.  This  sup- 
position is  supported  by  the  fact  that  the  central  apparatus 


144  MODIFICATIONS  OF  BEHAVIOR 

is  much  more  susceptible  to  the  effects  of  fatigue  than  the 
afferent  or  efferent  nerves.  It  is  not  a  fatal  objection  to  the 
theory  of  fatigue  that  the  response  falls  off  very  quickly. 
An  extreme  sensitiveness  may  result  from  a  certain  con- 
dition of  balance  which  a  slight  chemical  change  might 
overthrow  without  rendering  the  organism  insensitive  to 
stronger  stimuli.  A  very  high  degree  of  sensibility  is  as  a 
rule  very  easily  affected.  In  ourselves  the  ability  to  detect 
a  faint  odor  or  taste  is  exhausted  by  a  very  few  trials 
and  in  lower  organisms  where  sensitivity  is  often  exceed- 
ingly acute,  there  are,  as  we  might  expect,  much  greater 
fluctuations.  It  is  not  improbable  that  in  many  cases 
something  analogous  to  fatigue  may  take  place  in  the 
central  apparatus  in  the  pathway  between  the  afferent  and 
the  efferent  impulses.  The  animal  might  thus,  without  hav- 
ing either  its  receptors  or  its  motor  apparatus  appreciably 
affected,  fail  to  respond  in  the  same  way,  if  at  all,  to  stimuli 
which  at  first  brought  about  a  reaction. 

At  times  the  response  to  a  given  stimulus  may  be  increased 
instead  of  diminished  with  repeated  application.  Statke- 
witsch  found  that  Paramoecia  would  often  fail  to  respond  to 
weak  induction  shocks,  but  where  several  were  given  the  re- 
sults were  cumulative  and  the  animals  swam  toward  the 
cathode.  Planaria  after  a  period  of  rest  are  apparently  in  a 
condition  of  lowered  tonus  and  fail  to  react  in  the  usual 
manner  to  stimuli  but  if  the  stimuli  are  repeated  the  re- 
actions appear  and  for  a  time  may  increase  in  vigor.  An 
analogous  phenomenon  is  presented  in  the  responses  to  light 
of  Ranatra  and  fiddler  crabs  which  become  more  and  more 
energetic  with  longer  exposure  to  the  stimulus.  An  inter- 
esting instance  is  afforded  in  the  reaction  of  the  tentacle 
of  Cerianthus  to  repeated  tactile  stimuli.  Bohn  found 
that  the  tentacles  when  stimulated  became  flexed  toward 


DIFFERENT  KINDS  OF  RESPONSE  145 

the  mouth.  Early  in  the  morning  repeated  stimulation 
causes  the  tentacles  to  respond  with  increased  energy,  later, 
after  a  short  period  of  increase  of  responsiveness,  the  ten- 
tacles react  less  vigorously,  and  toward  evening  they  be- 
come insensible  after  a  few  stimulations. 

Very  similar  phenomena  w^ere  found  to  occur  in  the  annelid 
Serpula  when  stimulated  by  shadows.  The  responses  were 
variable;  some  individuals  would  show  an  increase  of  re- 
sponsiveness followed  by  a  decrease,  while  others  would  show 
a  diminution  from  the  beginning.  Bohn  has  attempted  to 
subsume  the  variations  of  responsiveness  to  a  given  stimulus 
under  one  general  *'law"  resting  on  a  physico-chemical 
basis.  All  stimulation  according  to  him  is  followed  at  first 
by  an  increase  and  then  by  a  decrease  of  reaction.  In  some 
cases  the  initial  increase  is  so  slight  as  not  to  attract  atten- 
tion and  we  apparently  get  a  falling  off  from  the  start.  In 
others  the  continued  increase  of  responsiveness  has  not 
been  followed  long  enough  to  discover  the  diminution  which 
must  come  sooner  or  later.  We  may  grant  the  latter  part 
of  Bohn's  contention,  but  the  first  is  much  more  difficult  to 
substantiate. 

DIFFERENT  KINDS  OF  RESPONSE  TO  A  GIVEN  STIMULUS 

Qualitative  variations  in  the  reactions  to  repeated  stimula- 
tions are  common.  We  have  seen  that  they  occur  in  an 
organism  as  low  in  the  scale  as  Stentor.  Hydra,  according 
to  Wagner,  when  repeatedly  stimulated  by  a  capillary  glass 
rod  at  sufficiently  long  intervals  will,  if  it  does  not  in  time 
ignore  the  stimuli  entirely,  come  to  respond  in  a  new  way; 
it  bends  to  one  side  until  the  tentacles  touch  the  bottom, 
then  loosens  the  foot  and  attaches  itself  in  another  locality. 
Striking  changes  in  the  response  to  a  given  stimulus  are 
furnished  by  the  reactions  of  sea  anemones.    The  anemone 

10 


146  MODIFICATIONS  OP  BEHAVIOR 

Aiptasia  when  a  drop  of  water  falls  on  its  disk  suddenly 
contracts.  To  several  subsequent  drops  there  is  no  further 
response.  Then  if  the  drops  continue  to  fall  the  animal 
contracts  still  further  and  draws  in  its  disk.  If  stimulated 
lightly  by  a  rod  the  anemone  contracts  strongly.  If,  when 
it  subsequently  extends,  it  is  again  stimulated  it  responds  in 
the  same  way,  and  continues  to  do  so  during  a  number  of 
trials.  Finally  the  anemone  as  it  extends  bends  over  in  a 
new  direction,  and  if  the  stimulus  persists  this  reaction  is 
repeated  several  times;  then  another  direction  of  extension 
is  tried  and  finally  if  the  stimulus  is  not  avoided  the  animal 
releases  its  foothold  and  crawls  into  a  new  locality. 

Anemones  have  various  methods  of  getting  rid  of  foreign 
bodies  on  the  disk,  or  even  morsels  of  food,  if  much  food  has 
already  been  taken.  The  procedure  in  Stoichactis  as 
described  by  Jennings  is  as  follows:  ''The  tentacles  bearing 
the  debris  or  the  rejected  food  body  collapse,  becoming  thin 
and  slender,  and  lying  flat  against  the  disk.  At  the  same  time 
the  disk  surface  in  this  region  begins  to  stretch,  separating 
the  collapsed  tentacles  widely.  A^  a  result  the  waste  mass  is 
left  on  a  smooth,  exposed  surface,  the  tentacles  here  having 
practically  disappeared — though  under  usual  conditions  they 
form  a  close  investment  almost  completely  hiding  the  surface 
of  the  disk.  Thus  the  waste  mass  is  fully  exposed  to  the 
action  of  waves  or  currents,  and  the  slightest  disturbance 
in  the  water  washes  it  off.  Under  natural  conditions  this 
must  usually  result  in  an  immediate  removal  of  the  debris. 
If  this  does  not  occur  at  once,  often  the  region  on  which 
the  debris  is  resting  begins  to  swell,  and  becomes  a  strongly 
convex,  smooth  elevation,  thus  rendeiing  the  washing  away 
of  the  mass  still  easier. 

"  But  the  process  may  go  much  farther.  If  the  debris  is 
not  removed  in  the  way  just  described,  new  reactions  set  in. 


DIFFERENT  KINDS  OF  RESPONSE  147 

If  the  mass  is  nearer  one  edge  of  the  disk  this  edge  usually 
begins  to  sink,  while  at  the  same  time  the  tentacles  between 
the  edge  and  the  waste  object  collapse  and  practically 
efface  themselves.  Thus  a  smooth,  sloping  surface  is  pro- 
duced and  the  waste  mass  slides -off  the  disk.  If  this  does 
not  occur  at  once,  after  a  little  time  the  region  l3dng  behind 
the  mass  (between  it  and  the  center  of  the  disk)  begins  to 
swell,  producing  a  high,  rounded  elevation,  with  tentacles 
plump  and  swollen.  The  waste  mass  is  now  on  a  steep 
slope,  and  is  bound  soon  to  slide  down  and  over  the  edge. 
Sometimes  by  a  continuation  of  this  process  the  entire  disk 
comes  to  take  a  strongly  inclined  position,  with  the  side 
bearing  the  debris  below.  Often  one  portion  of  the  edge  of 
the  disk  after  another  is  lowered  in  this  way,  till  all  the  w^aste 
matter  has  been  removed.  The  disk  then  resumes  its 
horizontal  position,  with  nearly  flat  or  slightly  concave 
surface."  If  the  edge  near  which  the  foreign  body  is  placed 
cannot  be  lowered  the  part  external  to  the  body  may  be 
raised  while  the  surface  toward  the  opposite  edge  is  depressed 
so  that  the  object  may  be  rolled  off  the  disk  in  another 
direction. 

In  Planaria,  according  to  Pearl,  repeated  mechanical 
stimulation  of  the  anterior  end  causes  the  worm  to  turn 
farther  away  at  each  succeeding  stimulus,  without  at  first 
causing  any  movements  of  locomotion.  After  a  time  the 
worm  jerks  back  vigorously,  bends  the  body  strongly  to  one 
side  and  then  extends  usually  toward  the  source  of  stimulus. 
Stronger  stimuli  soon  alter  the  general  physiological  condi- 
tion of  the  organism;  "the  animal  becomes  'stirred  up' 
generally,  moves  about  with  increased  rapidity,  its  sensitive- 
ness to  stimuli  becomes  diminished,  and  it  will  give  only  the 
negative  response  to  stimulation  of  the  anterior  end.  .  .  . 
One  may  get  totally  different  appearances  from  an  individual 


148  MODIFICATIONS  OF  BEHAVIOR 

which  has  been  'stirred  up'  from  what  are  seen  in  the  case 
of  one  which  is  in  the  normal  condition/' 

In  the  earthworm  repeated  stimuU  give  rise  to  a  great 
variety  of  reactions  which  have  been  classified  by  Jennings 
as  follows: 

''  (a)  The  state  of  rest,  in  which  the  worm  does  not 
react  readily  to  slight  stimuli,  such  as  a  touch  with  the  tip 
of  a  glass  rod. 

''(b)  A  state  of  moderate  activity,  in  which  a  touch  at  the 
anterior  end  causes  movement  backward;  at  the  posterior 
end  movement  forward,  while  lateral  stimuli  (in  the  anterior 
region)  cause  turning  away  from  the  side  stimulated. 

"(c)  A  state  of  excitement,  after  repeated  stimuli,  in 
which  the  animal  persists  in  the  direction  of  movement  once 
begun,  merely  stopping  for  a  few  seconds  when  stimulated 
at  the  end  which  is  advancing. 

"(d)  A  state  of  greater  excitement,  in  which  stimuli  merely 
cause  the  animal  to  hasten  its  movements  in  the  direction  in 
which  it  has  started,  without  regard  to  the  localization  of 
the  stimulus. 

"(e)  A  state  of  still  greater  excitement,  after  long-con- 
tinued and  intense  stimulation.  Now  the  worm  responds 
to  a  stimulus  at  the  anterior  end,  that  would  in  a  resting 
worm  cause  only  a  comparatively  slight  reaction,  by  a  rapid 
'right-about-face.'  The  body  is  suddenly  doubled  at  its 
middle,  so  that  the  anterior  and  posterior  halves  become 
parallel,  with  the  two  ends  pointing  in  the  same  direction, 
then  the  posterior  half  is  quickly  whipped  about,  so  that 
the  whole  worm  is  again  straight,  but  is  facing  the  opposite 
direction  from  that  in  which  it  was  pointed  before  the 
reaction.   .    .    . 

"  (f)  A  state  of  still  more  intense  excitement,  after  repeated 
strong  stimulation  that  is  of  such  a  character  as  to  actually 


DIFFERENT  KINDS  OF  RESPONSE  149 

injure  the  tissues.  The  worm  now  responds  to  a  repetition 
of  the  stimulus  (and  often  when  the  new  stimulus  is  only 
slight)  by  lifting  the  anterior  fourth  of  the  body  into  a 
vertical  position,  and  waving  it  about  in  a  frantic  manner. 
-This  behavior  is  usually  alternated  with  the  right-about-face 
reactions,  and  with  persistent  rapid  crawling  forward  and 
backward.  ..."  These  reactions  do  not  exhaust  the 
list  but  are  simply  more  typical  features  of  an  almost  endless 
variety  of  modifications. 

The  causes  of  the  change  of  response  to  a  given  stimulus 
may  be  many.  The  factor  of  fatigue  is  doubtless  an  im- 
portant one,  especially  in  the  nervous  centers.  An  afferent 
fiber  may  be  connected  with  several  motor  pathways,  so 
that  slight  variations  in  the  ease  with  which  impulses  may 
travel  along  certain  lines  may  determine  the  direction  of 
motor  discharge.  A  very  slight  degree  of  fatigue  of  any 
pathway  might  then  be  a  cause  of  a  change  of  response  after 
one  or  a  very  few  repetitions.  An  increase  of  transmitting 
function  as  a  result  of  previous  exercise  may  also  play  its 
part,  so  that  we  may  say  that  the  factors  which  produce  an 
increase  or  decrease  of  responsiveness  also  effect  the  replace- 
ment of  one  response  by  another  of  a  different  kind.  In  the 
one  case  they  modify  the  intensity  of  the  reaction,  in  the 
other  they  act  so  as  to  switch  off  the  action  to  other  lines. 

The  fluctuations  of  tonus  here  and  there  in  the  nervous  and 
muscular  systems  afford,  as  Uexkiill  has  shown,  a  mechanism 
by  which  the  responses  of  the  organism  may  be  varied  to  an 
indefinite  degree.  Every  act  of  an  organism  alters  the  dis- 
tribution of  tonus  in  its  body,  and  the  way  in  which  the  organ- 
ism responds  to  the  next  stimulation  is  naturally  affected 
by  this  circumstance.  Then  variations  in  assimilation, 
respiration,  excretion  and  other  functions  are  constantly  inter- 
fering directly  or  indirectly  with  the  outcome.    There  is 


150  MODIFICATIONS  OF  BEHAVIOR 

therefore  no  occasion  for  wonder  if  the  organism  responds  in 
different  ways  to  a  given  stimulus.  The  wonder  would  be 
if  it  always  responded  in  the  same  way. 

INFLUENCE  OF  INTERNAL  FACTORS  ON  BEHAVIOR 

That  behavior  of  organisms  in  general  should  be  greatly 
influenced  by  their  internal  condition  is  obvious.  Hunger 
and  repletion  influence  the  activity  of  animals,  even  in  the 
lowest  forms.  Even  the  white  blood  corpuscles  when  they 
have  ingested  a  number  of  bacteria  refuse  to  take  in  any 
more.  Paramoecium,  while  it  may  not  reject  food  swept 
into  its  gullet,  behaves  differently  when  well  fed,  and  Stentor, 
as  we  have  seen  in  a  previous  chapter,  takes  in  objects 
when  it  is  hungry  which  at  other  times  are  rejected.  Hy- 
dras when  hungry  eagerly  take  in  food,  but  they  are  quite 
indifferent  to  it  when  well  fed.  If  starved  for  some  time 
they  become  more  active,  extend  and  contract  the  tentacles 
and  body,  and  move  about  in  various  directions  as  if  to 
increase  their  chances  of  coming  in  contact  with  food.  The 
jelly-fish  Gonionemus  when  hungry  swims  about  actively, 
frequently  coming  to  the  surface  and  settling  slowly  through 
the  water,  with  its  tentacles  extended  to  catch  its  food.  After 
a  hearty  meal  the  jelly-fish  is  more  frequently  at  rest  and  has 
its  tentacles  contracted. 

The  effect  of  hunger  on  the  movements  of  sea  anemones 
is  very  striking.  When  starving,  anemones  will  often  take 
in  such  things  as  filter-paper  and  stones  which  they  reject 
under  ordinary  conditions,  as  is  well  shown  in  the  experiments 
of  Torrey  on  Sagartia.  Weak  food  stimuli  produced  by 
giving  the  anemones  filter-paper  soaked  with  dilute  crab 
juice  produce  at  first  the  food  reaction,  but  in  a  little  while 
the  animal  no  longer  responds  (Nagel,  Parker).  If  meat 
is  offered  the  food  taking  activities  continue  for  a  long  time. 


INFLUENCE  OF  INTERNAL  FACTORS  151 

but  eventually  become  weaker  and  may  later  be  replaced  by 
movements  of  rejection.  That  it  is  not  the  mere  filling  of  the 
digestive  cavity  alone  that  determines  the  change  of  be- 
havior is  showTi  in  an  experiment  of  Jennings  on  Aiptasia, 
in  which  the  digestive  cavity  was  filled  with  filter-paper. 
When  so  filled  that  pieces  of  paper  were  repeatedly  disgorged 
the  anemones  continued  to  take  in  new  pieces.  The  meta- 
bolic conditions  of  the  organism  are  therefore  a  determining 
factor  in  its  behavior  toward  food. 

To  a  certain  extent  though,  the  food  taking  is  influenced 
by  the  previous  local  stimulations  of  particular  parts  of  the 
body.  Nagel  and  Parker  have  found  in  Adamsia  and 
Metridium  that  the  tentacles  of  a  certain  part  of  the  body 
after  being  stimulated  several  times  with  filter-paper  soaked 
in  meat  juice  will  refuse  to  carry  it  toward  the  mouth.  If 
now  the  soaked  paper  is  offered  to  other  tentacles  they  give 
the  usual  food  taking  reaction.  Similai'  behavior  was  ob- 
served by  Jennings  in  Aiptasia,  who  found  also  that  if  one  set 
of  tentacles  had  carried  several  pieces  of  meat  to  the  mouth 
until  they  refused  to  carry  it  longer,  the  other  tentacles, 
while  they  might  still  carry  the  meat,  would  cease  to  respond 
much  sooner  than  they  would  if  the  animal  had  not  already 
received  food.  The  animal  according  to  Jennings  "is  a 
unit  so  far  as  hunger  and  satiety  are  concerned." 

The  effects  of  hunger  and  satiety  on  the  behavior  of  higher 
forms  are  so  general  and  so  familiar  that  we  need  not  pursue 
the  subject  further.  As  regards  food  animals  in  general  are 
self  regulating  mechanisms,  and  if  under  certain  conditions 
an  injurious  quantity  of  food  is  devoured,  or  material  which 
is  unwholesome  is  selected,  the  exceptional  behavior  only 
serves  to  emphasize  the  rule  that  the  food  taking  of  animals 
is  on  the  whole  pretty  adequately  adjusted  to  their  needs. 
In  higher  forms  this  function  is  conscious  and  voluntary, 


152  MODIFICATIONS  OF  BEHAVIOR 

but  carry  the  process  down  the  scale  of  life  and  it  begins  to 
take  on  the  character  of  other  organic  regulations  such  as  we 
find  in  the  tissue  cells  of  our  own  bodies;  for  even  there  the 
intake  and  assimilation  of  nutriment  is  regulated  as  in  free 
organisms.  In  none  of  the  animals  we  have  described  is 
there  evidence  of  intelligence  in  the  selection  of  food.  The 
choice  made  is  more  readily  explained  in  terms  of  what 
Bohn  calls  differential  sensibility.  The  organism  is  so  con- 
structed that  it  responds  to  certain  kinds  of  stimuli  in  one 
way  and  to  other  kinds  of  stimuli  in  a  different  way.  But  it 
also  has  the  faculty  of  responding  to  the  same  stimulus  in 
different  ways  at  different  times.  Here  it  may  be  assumed 
that  the  different  response  is  the  effect  of  changes  of  internal 
conditions  which  alter  the  irritability  of  the  animals.  The 
stimuli  set  up  by  the  presence  of  food  in  the  digestive  tract, 
the  secretory  activity  of  the  cells,  the  processes  of  absorption 
and  assimilation  going  on  throughout  the  body  afford  a 
complex  of  influences  affecting  the  neuro-muscular  mechan- 
ism and  naturally  modifying  its  action. 

In  the  sea  anemones,  contact  tends  to  set  in  operation 
one  or  the  other  of  two  mechanisms — one  involved  in  taking 
in  food,  the  other  in  its  rejection.  These  mechanisms  are 
mutually  inhibitory  and  often  there  is  a  struggle  between 
them  resulting  in  a  hesitation  or  vacillation  in  the  response 
of  the  organism.  Internal  conditions  act  as  a  sort  of  brake 
on  one  or  the  other  mechanism  and  thus  change  the  nature 
of  the  response  to  a  particular  external  stimulus. 

This  type  of  behavior  which  has  often  been  assumed  to 
indicate  consciousness  if  not  intelligence  is  not  anything 
which  cannot  be  accounted  for  on  the  basis  of  reflex  action. 
It  is  a  type  of  behavior  which  is  wide  spread,  in  fact  probably 
coextensive  with  animal  life.  If  it  is  a  criterion  of  intelli- 
gence we  must  assume  that  all  animals  are  intelligent  and 


HABITS  153 

not  only  that,  but  that  the  cells  of  our  bodies  are  intelligent 
likewise. 

The  food  taking  activities  of  animals  are  admirably  adapted 
to  illustrate  the  essential  unity  of  behavior  throughout  the 
animal  kingdom.  These  activities  have  their  roots  in  the 
fundamental  processes  of  organic  life.  Intelligent  food 
getting  is  based  on  fundamental  instinctive  tendencies, 
in  fact  is  but  a  way  in  which  these  tendencies  are  shaped 
in  order  a  little  more  effectually  to  reach  the  goal.  The 
still  hunt  of  the  hungry  lion,  the  expanded  tentacles  of  the 
hungry  anemone  or  jelly-fish,  the  restless  activity  of  the 
starving  protozoan  are  all  expressions  of  a  common  state, 
and  are  dictated  by  a  common  need. 

HABITS 

Even  very  primitive  animals  may  acquire  ways  of  acting 
or  responding  to  stimuH  through  the  effects  of  their  previous 
experience.  These  habits,  if  we  may  call  them  such,  are 
usually  not  very  permanent  and  tend  to  wear  away  after 
the  determining  cause  is  removed.  Jennings  has  observed 
that  specimens  of  the  anemone  Aiptasia  annulata  which 
live  in  crevices  among  the  rocks  where  in  expanding  they 
have  to  bend  their  bodies  in  an  irregular  way  stiU  retain  their 
irregular  movements  in  expanding  after  they  have  been 
removed  from  their  original  habitats.  This  led  him  to  make 
some  observations  on  normal  anemones  subjected  to  re- 
peated stimulations.  In  one  case  "an  individual  attached 
to  a  plane  horizontal  glass  surface  was  bent  in  extension 
far  over  to  the  left.  Stimulating  it  repeatedly,  it  con- 
tracted at  each  stimulation,  then  bent,  in  extending,  again 
to  the  left.  This  continued  for  fifteen  stimulations,  one 
succeeding  another  as  soon  as  the  animal  had  become  fully 
extended.     At  the  next  contraction  the  animal  turned  and 


154  MODIFICATIONS  OF  BEHAVIOR 

bent  over  to  the  right.  Now  when  stimulated  it  contracted 
as  before,  then  bent  regularly,  in  extending,  over  to  the 
right.  It  seemed  to  have  acquired  a  new  habit — bending 
to  the  right  instead  of  to  the  left.'^  Observation  showed 
that  the  bend  of  the  body  persisted  in  the  contracted  as  well 
as  the  extended  state,  and  that  all  parts  extended  propor- 
tionately and  thus  led  to  a  repetition  of  the  previous  action. 
The  habit  of  action  in  this  case  had  a  persistent  structural 
basis  in  the  bend  of  the  body.  If  now  the  anemone  was 
caused  to  contract  very  strongly  in  all  parts  so  that  it  was 
no  longer  bent  to  one  side  in  the  contracted  state  it  would 
lose  its  habit  of  bending  when  it  subsequently  expanded. 
The  role  of  the  nervous  system  in  this  case  is  a  very  doubtful 
one  and  it  is  not  improbable  that  the  so-called  habit  is 
merely  a  result  of  purely  mechanical  factors.  The  same 
may  also  be  true  of  the  habit  of  irregular  bendings  acquired 
as  a  result  of  living  in  crevices  between  the  rocks,  which 
might  be  compared  to  the  difficulty  experienced  by  a  person 
whose  body  and  limbs  have  been  confined  for  a  long  time 
in  any  one  position  in  making  new  movements. 

Jennings  has  performed  numerous  experiments  on  the 
formation  of  righting  habits  in  the  starfish  Asterias  forrei. 
Starfish  when  placed  on  their  backs  have  several  methods 
of  turning  over.  Different  individuals  have  their  personal 
peculiarities  in  this  as  in  other  respects,  and  in  most  speci- 
mens there  is  a  tendency  to  use  one  particular  ray  or  set  of 
rays  upon  which  to  turn.  These  differences  as  Moore 
has  pointed  out  may  be  due  to  inequalities  in  the  size  of 
^'Vy  the  arms,  injuries  to  certain  arms,  or  any  initial  twist  an 
arm  may  have  had  due  to  its  previous  position.  Before 
any  attempt  to  train  a  starfish  was  made  the  animal  was 
put  through  a  set  of  tests  to  determine  the  rays  most  com- 
monly employed  in  the  righting  reaction.     After  this  the 


RHYTHMS  IN  BEHAVIOR  155 

starfish  was  prevented  from  using  these  rays  and  was  al- 
lowed to  turn  only  on  those  rays  which  it  was  least  prone 
to  employ.  If,  after  a  number  of  trials,  the  starfish  came 
to  employ  the  latter  most  frequently  it  would  be  fair  to 
conclude  that  the  animal  had  acquired  a  new  habit  of  turn- 
ing. After  one  or  two  days'  training  most  of  the  individ- 
uals experimented  on  came  to  turn  upon  rays  which  they 
were  at  first  disinclined  to  use.  The  effect  of  the  training 
was  not  manifest,  however,  after  twenty-four  hours.  In 
specimens  "trained"  for  a  week  or  more  the  habit  formed 
persists,  according  to  Jennings,  for  twenty-four  hours  or 
even  several  days.  The  data  obtained  on  this  subject 
were  not  extensive,  and  when  we  consider  several  sources 
of  error  involved,  not  entirely  convincing;  further  work 
would  be  necessary  before  we  could  safely  conclude  that 
starfish  form  lasting  habits. 

How  shall  habit  formation  in  the  starfish  be  interpreted? 
Moore  has  sho\sTi  that  irritation  of  an  arm  inhibits  its  ac- 
tivity and  causes  the  starfish  to  use  other  arms  in  the  right- 
ing movements;  and  the  operation  of  handling  the  animal 
during  its  "lessons"  so  as  to  prevent  a  certain  arm  from 
being  used  might  produce  a  similar  effect.  There  is  no  evi- 
dence that  there  is  any  element  of  association  involved,  and 
it  is  not  quite  clear  whether  the  basis  of  the  habit  lies  in 
the  nervous  system  or  in  other  parts  of  the  organization. 

RHYTHMS  IN  BEHAVIOR 

Georges  Bohn  has  the  merit  of  having  discovered  that 
the  rhythms  of  activity  which  are  produced  in  many  animals 
by  regularly  recurring  external  conditions  such  as  the  alter- 
nation of  day  and  night  and  the  periods  of  low  and  high  tide 
may  persist  for  some  time  after  the  animals  are  withdrawn 
from  the  direct  influence  of  the  outer  periodic  changes. 


156  MODIFICATIONS  OF  BEHAVIOR 

Convoluta  roscoffensis,  a  small  green  turbellarian  worm 
which  lives  in  sandy  beeches  overflowed  by  the  tide,  makes 
periodical  depth  migrations  in  the  sand.  At  low  tide  the 
worms  come  to  the  surface  where  they  form  a  green  coat 
upon  the  shore.  When  the  tide  comes  in  the  worms  de- 
scend and  thus  avoid  the  shock  of  the  waves.  Bohn  found 
that  the  same  periodic  migrations  occurred  in  specimens 
which  were  taken  from  the  shore  and  kept  in  an  aquarium. 
If  the  worms  were  placed  in  a  tube  of  sand  a  green  ring  could 
be  seen  to  rise  and  descend  synchronously  with  the  ebb 
and  rise  of  the  tide.  These  rhythms  persisted  for  several 
days  after  the  worms  were  removed  from  the  beach.  Keeble 
has  compared  the  vertical  movements  of  Convolutas  kept 
in  the  laboratory  with  the  movements  of  specimens  on  the 
beach,  and  states  that  ^'for  eight  successive  tides  the 
animals  in  the  laboratory  maintain  their  rhythm,  synchron- 
ous with  the  ebb  and  flow  of  the  waters  over  the  roscoffensis 
zone:  then,  though  the  rhythmic  movement  up  and  down 
may  continue,  its  temporal  periodicity  loses  precision,  and, 
finally,  the  rhythm  is  worn  down." 

A  parallel  phenomenon  was  discovered  by  Bohn  in  the 
diatom  Pleurosigma.  When  the  sea  retired  these  diatoms 
were  observed  to  form  a  brown  scum  over  the  sand.  When 
the  tide  came  in  the  diatoms  descended.  Placed  in  an 
aquarium  they  performed  regular  migrations  for  several 
days  in  accordance  with  the  tidal  rhythms.  This  periodic- 
ity failed  to  manifest  itself  in  darkness  and,  therefore, 
according  to  Bohn,  depends  upon  variations  in  phototaxis 
instead  of  the  response  to  gravity.  Bohn  has  reported 
that  a  curious  tidal  rhythm  occurs  in  the  small  gastropod, 
Littorina  rudis,  which  lives  upon  the  rocks  where  it  is  only 
occasionally  wet  by  the  waves.  At  low  tide  these  molluscs 
withdraw  into   their  shells   and   remain  inactive.     When 


RHYTHMS  IN  BEHAVIOR  157 

the  spray  from  the  incoming  tide  begins  to  moisten  them 
they  emerge  and  begin  crawling  about.  Their  phototaxis 
changes  with  the  tide,  becoming  negative  when  the  tide 
is  high  and  positive  when  it  is  low.  Bohn  placed  Littorina 
in  an  aquarium  and  found  that  for  several  days  they  under- 
went changes  in  their  phototactic  responses  parallel  with 
those  of  specimens  upon  the  rocks.  Similar  experiments 
have  been  made  by  Morse  on  a  species  of  Littorina  on  the 
New  England  coast,  but  he  failed  to  obtain  any  evidence 
of  a  tidal  rhythm.  And  more  recently  Haseman  has  in- 
vestigated the  tidal  rhythms  of  several  species  of  Littorina, 
none  of  which  showed  any  rythmical  movements  independ- 
ent of  the  direct  influence  of  the  tides. 

Among  actinians  Bohn  has  found  in  many  cases  daily 
rhythms  due  to  the  alternation  of  Hght  and  darkness  super- 
posed upon  tidal  rhythms.  Both  of  these  rhythms  are 
influenced  greatly  by  the  nature  of  the  habitat  in  which  the 
anemones  live.  Individuals  situated  rather  high  upon  the 
rocks  and  living  therefore  under  strongly  contrasted  con- 
ditions in  high  and  in  low  tide  exhibit  tidal  rhythms  to  a 
marked  degree;  whereas  those  which  hve  at  depths  in  which 
they  are  little  affected  by  the  waves  show  little  or  no  in- 
fluence of  the  tide.  Alternation  of  day  and  night  affects 
anemones  to  a  greater  or  less  degree  in  all  habitats;  its 
influence  is  compHcated  by  many  factors,  such  as  degree  of 
exposure  to  the  sun,  depth  of  water,  shock  of  waves,  tem- 
perature, purity  of  the  water,  and  various  other  causes. 
The  daily  rhythms  differ  greatly  in  anemones  from  different 
local  situations,  according  to  the  influences  to  which  the 
animals  are  adapted.  These  rhythms  with  their  various 
characteristics  peculiar  to  different  habitats  were  found  to 
persist  for  several  days  in  aquaria,  but  they  gradually  wore 
away. 


158  MODIFICATIONS  OF  BEHAVIOR 

Among  higher  invertebrates  tidal  rhythms  have  been 
observed  by  Drzewina  in  the  hermit  crab  Clibanarius  miS' 
anthropus.  Specimens  were  collected  at  various  times  and 
kept  in  aquaria  one  end  of  which  was  shaded.  Nearly  all 
of  the  individuals  manifested  a  positive  phototaxis  when  the 
tide  was  high,  and  became  negative  when  the  tide  was  low. 
These  regular  changes  persisted  in  the  lot  which  was  kept 
longest,  for  three  weeks. 

We  have  in  these  periodic  variations  of  behavior  habits 
of  action  in  relation  to  different  influences  of  the  environ- 
ment which  have  been  acquired  by  the  experience  of  the 
organism.  These  habits  have  probably  not  been  acquired 
tlirough  intelligence  in  most  cases  and  certainly  not  in  the  case 
of  the  diatoms,  and  it  does  not  seem  improbable  that  all  of 
them  may  be  dependent  upon  some  general  modifications 
of  the  organism  as  a  whole  rather  than  upon  merely  the 
mechanism  of  response  to  stimuH. 

HABITS  VARIOUSLY  CAUSED 

Habits  may  be  due  to  a  variety  of  causes.  They  may 
result  from  the  repeated  displacements  of  given  structures. 
They  may  depend  upon  artificially  induced  organic  rhythms. 
They  may  arise  as  the  outcome  of  intelligently  formed  acts. 
What  closely  simulates  a  temporary  habit  may  be  the  physi- 
ological effect  of  the  summation  of  stimuli.  An  action 
system  put  in  operation  one  or  more  times  by  a  given  stimu- 
lus may,  up  to  a  certain  point,  become  increasingly  respon- 
sive to  that  stimulus.  There  is  an  increase  of  the  tonus  of 
the  parts  concerned.  If  in  righting  itself  a  starfish  comes 
to  employ  a  particular  ray,  that  ray  as  a  result  of  its  exercise 
may  respond  with  increased  readiness.  Whether  the  tem- 
porary habits  formed  in  the  starfish  studied  by  Jennings 
were  due  in  part  at  least  to  an  increase  of  tonus  as  the  result 


DECEPTIVE   APPEARANCES  OF  INTELLIGENCE  159 

of  previous  activity  I  do  not  pretend  to  say;  but  the  case  is 
cited  as  an  illustration  of  a  possible  way  in  which  a  certain 
class  of  habits  may  be  interpreted. 

Habit  formation  may  be  shown  in  the  action  of  individual 
organs  of  our  bodies.  The  stomach  according  to  the  re- 
searches of  Pawlow  readily  acquires  habits,  and  the  influence 
of  habit  in  the  functioning  of  the  intestines  is  generally 
familiar.  Between  such  habit  formation  and  the  increase 
of  an  organ  through  action  there  is  much  in  common,  and 
between  these  phenomena  and  the  adaptive  changes  of  an 
organ  in  relation  to  its  condition  of  stimulation  there  is 
doubtless  a  fundamental  kinship.  \Miat  Roux  has  called 
the  "overcompensation  of  what  is  used"  is  a  principle  which 
probably  manifests  itself  in  all  these  cases. 

DECEPTIVE  APPEARANCES  OF  INTELLIGEWCE 

There  are  a  great  many  cases  which  have  been  adduced  as 
indicating  intelligence  and  even  a  simple  form  or  reasoning 
which  may  be  explained  like  the  phenomena  we  have  de- 
scribed. It  is  frequently  difficult  to  distinguish  acts  which 
are  instinctive  from  those  which  an  animal  has  learned  to 
perform,  and  it  is  well  in  general  to  be  guided  by  the  prin- 
ciple enunciated  by  Lloyd  Morgan,  which  is  a  sort  of  special 
case  of  the  law  of  parsimony — namely,  that  "  In  no  case  may 
we  interpret  an  action  as  the  outcome  of  the  exercise  of  a 
higher  psychical  faculty,  if  it  can  be  interpreted  as  the  out- 
come of  one  which  stands  lower  in  the  psychological  scale.'' 
One  of  the  cases  most  suggestive  of  the  power  of  forming 
associations  in  the  Coelenterates  is  recorded  by  Fleure  and 
Walton  whose  account  is  as  follows: 

"We  have  given  a  specimen  of  Actinia  a  scrap  of  filter- 
paper  about  once  in  twenty-four  hours,  placing  it  on  the 
same  tentacles  each  time.    As  a  general  rule  the  fragment  was 


160  MODIFICATIONS  OF  BEHAVIOR 

carried  by  the  tentacles  to  the  mouth  and  there  swallowed, 
to  be  ejected  as  inedible  after  a  longer  or  shorter  period. 
After  a  few  days,  the  number  varying  in  different  individuals 
from  two  to  five  days,  the  fragment  is  no  longer  swallowed 
and,  in  about  an  other  two  days,  the  tentacles  will  no  longer 
take  hold  of  it.  This  procedure  is  more  regular  in  the  case 
of  pellets  of  paper  than  in  the  case  of  India  rubber,  in  which 
results  were  very  variable.  The  results  seem  to  indicate  a 
certain  amount  of  persistence  of  impressions,  when  the  latter 
are  received  several  times  in  succession  at  short  intervals. 

"The  first  impression  which  persists  is  one  in  the  mouth 
region  leading  to  refusal  of  the  pellet.  As  this  is  strengthened 
the  sequence  is  further  abbreviated,  an  inhibitory  stimulus 
would  seem  to  proceed  from  the  mouth  to  the  tentacles 
preventing  them  from  gripping,  when  the  stimulation  due  to 
contact  with  the  filter-paper  has  passed  from  them  to  the 
mouth. 

"  A  further  point  of  interest  is  that  what  does  persist  seems 
to  remain  a  property  of  the  tentacles  affected,  and  of  that 
part  of  the  mouth  directly  related  to  them.  It  does  not 
appear  to  be  a  possession  of  the  entire  animal,  for  other 
tentacles,  on  the  opposite  side  for  instance,  can  be  tricked 
subsequently,  at  any  rate  once  or  twice,  before  they  too 
exhibit  the  inhibitory  reaction."  The  effects  of  the  ex- 
perience were  found  to  be  lost  after  from  six  to  ten  days. 

Whether  or  not  we  have  in  this  instance  the  formation  of 
a  new  association,  as  the  behavior  of  the  anemone  seems 
to  indicate,  is  not  entirely  certain.  It  is  possible  that  the 
seat  of  the  change  of  behavior  is  in  the  tentacles  alone. 
Allabach  has  shown  that  after  the  tentacles  of  Metridium 
have  responded  to  a  stimulus  a  few  times  their  production 
of  mucus  becomes  much  diminished  and  this  probably 
affects   their   subsequent   activity.     If   this   factor   would 


DECEPTIVE  APPEARANCES  OF  INTELLIGENCE   161 

modify  the  irritability  of  the  tentacles  for  some  time  it 
might  explain  the  change  of  behavior.  This  may  not  be 
the  true  explanation  of  the  phenomenon,  but  it  will  serve 
to  show  how  careful  we  must  be,  in  studying  the  behavior 
of  lower  organisms,  about  inferring  the  presence  of  associa- 
tive memory.  There  have  been  almost  no  studies  of  the 
power  of  association  in  the  Ccelenterates,  where  the  various 
possibilities  of  error  have  been  carefully  excluded. 

Darwin  in  his  work  on  earthworms  attributes  a  certain 
degree  of  intelligence  to  these  creatures  on  account  of  their 
pecuHar  habits  of  plugging  up  their  burrows  with  dead  leaves. 
The  worms  pull  in  the  leaves  of  the  linden  by  their  tips,  while 
the  leaves  of  the  rhododendron  which  are  smaller  at  the  base 
are  pulled  in  by  the  petiole.  Pine  needles  which  frequently 
occur  in  pairs  with  a  common  base  are  not  seized  by  the 
small  end,  which  would  cause  difficulity  in  getting  both 
needles  into  the  hole,  but  by  the  enlargement  at  the  basal  end. 
Darwin  gave  the  worms  triangles  of  paper  and  found  that 
they  usually  seized  these  by  the  most  acute  angle  in  carrying 
them  to  their  burrows.  The  conclusions  of  Darwin  that  the 
behavior  of  the  earthworms  indicates  a  certain  degree  of 
intelligence  was  a  very  natural  one.  Hanel,  however, 
who  has  repeated  and  verified  Darwin^s  experiments  and 
performed  a  number  of  others,  finds  no  ground  for  assuming 
any  intelligence  in  the  earthworm  and  ascribes  the  behavior 
of  the  animal  to  a  series  of  more  or  less  complex  reflexes  in 
relation  to  the  form  and  chemical  nature  of  the  objects 
drawn  in.  There  is  no  evidence  of  profiting  by  experience 
in  the  earthworm's  behavior  and,  however  complex  the  acts 
performed,  there  is  nothing  that  is  thus  far  known  that 
precludes  us  from  considering  them  as  belonging  entirely 
to  the  reflex  type. 


11 


162  MODIFICATIONS  OF  BEHAVIOR 

BIBLIOGRAPHY 

BoHN,  G.  Sur  les  mouvements  oscillatoires  des  Convoluta  roscof- 

fends.     C.  r.  Ac.  Sci.,  Paris,  137,  576,  '03. 

Les  Convoluta  roscoffensis  et  la  theorie  des  causes  actuelles. 

Bull.  Mus.  Hist.  Nat.,  9,  352,  '03. 

Intervention  des  influences  pass^es  dans  les  mouvements  actuels 

d'un  animal.     C.  r.  Soc.  Biol.,  Paris,  56,  789,  '04. 

Periodicity  vital  des  animaux  soumis  aux  oscillations  du  niveau 

des  hautes  mers.     C.  r.  Ac.  Sci.,  Paris,  139,  610,  '04. 

Oscillations  des  animaux  littoraux  synchrones  des  mouvements 

de  la  mar^e.     Ibid..  139,  646,  '04. 

Mouvements  de  manege  en  rapport  avec  les  mouvements  de  la 

mar^e.     C.  r.  Soc.  Biol.,  Paris,  57,  297,  '04. 

Les  causes  actuelles  et  les  causes  pass6es.     Rev.  Scientif.,  3,  353 

and  389,  '05. 

Le  rhythme  nycth^m^ral  chez  les  actinies.     C.  r.  Soc.   Biol., 

Paris.,  62,  473,  '07. 

La  persistance  du  rhythme  des  marges  chez  I'Actinia  equina. 

Ibid.,  61,  661,  '06. 

Introduction    k    la    psychologie    des    animaux   k    symmetrie 

rayonn^e.'  1,  Les  6tats  physiologiques  des  actinies.     Bull.  Inst. 

G^n.  Psych.,  7,  81  and  135,  '07. 

De  I'acquisition  des  habitudes  chez  les  6toiles  de  mer.  C.  r. 

Soc.  Biol.,  Paris,  64,  277,  532,  633,  '08. 

Introduction  k  la  psychologie  des  animaux  k  symmetrie  rayon- 

n^e.   2,    Les   essais  et  erreurs  chez  les  6toiles  de  mer  et  les 

ophiures.     Bull.  Inst.  Gen.  Psych.,  8,  21,  '08. 

La  naissance  de  I'intelligence,  Paris,  '09. 
Darwin,  C.     The  Formation  of  Vegetable  Mould  through  the  Action 

of  Worms,  with  Observations  on  Their  Habits.     N.  Y.,  '83. 
Fleure,  H.  J.,  and  Walton,  C,  L.     Notes  on  the  Habits  of  Some 

Sea-Anemones.     Zool.  Anz.,  31,  212,  '07. 
Ghinst,  van  der.     Quelques  observations  sur  les  actinies.     Bull. 

Inst.  G^n.  Psych.  Paris,  6,  267,  '06. 
Glaser,  0.  C.     Movement  and  Problem  Solving  in  Ophiura  brevi- 

spina.    Jour.  Exp.  Zool.,  4,  203,  '07. 
Hargitt,    C.    W.     Experiments   on   the    Behavior   of   Tubicolous 

Annelids.     Jour.  Exp.  Zool.,  3,  295,  '06. 

Further  Observations  on  the  Behavior  of  Tubicolous  Annelids. 

Ibid.,  7,  157,  '09. 


DECEPTIVE  APPEARANCES  OF  INTELLIGENCE   163 

Jennings,  H.  S.     Contributions  to  the  Study  of  the  Behavior  of 

Lower  Organisms.     Carnegie  Inst.  Pubs.,  Wash.,  *04. 

Modifiability    in    Behavior.    1.    Behavior   of    Sea    Anemones. 

Jour.  Exp.  ZooL,  2,  447,  '05. 

The  Method  of  Regulation  in  Behavior  and  in  Other  Fields, 

Ibid.,  2,  473,  '05. 

Modifiability  in  Behavior.     2.  Factors  Determining  Direction 

and   Character  of   Movement  in    the   Earthworm.     Ibid.,  3, 

435,  '06. 

Behavior  of  Lower  Organisms,  N.  Y.,  '06. 

Behavior  of  the  Starfish,  Asterias  forrei  de  Loriol.     Univ.  of 

Cahf.    Pubs.  Zool.  4,  53,  '07. 
Kbeble,    F.    Plant    Animals.    A    Study    in     S5rmbiosis.    Cam- 
bridge, '10. 
Pearl,  R.    The  Movements  and  Reactions  of  Fresh-Water  Plan- 

arians.     Quart.  Jour.  Mic.  Sci.,  46,  509,  '03. 
Pbeyer,  W.    Ueber  die  Bewegungen  der  Seesterne.    Mitth.  a.  d. 

zool.  Stat,  zu  Neapel,  7,  27,  and  191,  '86. 
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'03;  46,  1,  '04;  46,  372,  '04;  49,  307,  '07;  50,  168,  '08. 

Umwelt  und  Innenwelt  der  Tiere,  BerHn,  '09. 
Washburn,  M.  F.  The  Animal  Mind,  N.  Y.,  '09. 
Yerkes,  a.  W.     Modifiability  of  Behavior  in  Hydroides  dic^Uhus, 

Jour.  Comp.  Neur.  Psych.,  16,  441,  '06. 


CHAPTER  VIII 

PLEASURE,  PAIN,  AND  THE  BEGINNINGS  OF 
INTELLIGENCE^ 

"Apprehensio  sensitiva  non  attingit  ad  communem  rationem 
boni,  sed  ad  aliquod  bonum  particulare,  quod  est  delectabile.  Et 
ideo  secundum  appetitum  sensitivum,  qui  est  in  animalibus,  opera- 
tiones  quaeruntur  propter  delectionem." — Thomas  Aquinas,  Summa 
Theologica. 

Psychologists  nowadays  with  comparatively  few  exceptions 
agree  in  regarding  intelligence  not  as  a  faculty  standing  in 
sharp  contrast  to  instinct,  as  was  formerly  taught,  but  as  one 
resting  on  a  foundation  of  instinct,  and  gradually  growing 
out  of  behavior  of  the  purely  instinctive  type.  The  term 
intelHgence  is  used  here  in  the  wider  sense  as  embracing  all 
those  forms  of  profiting  by  experience  through  the  formation 
of  associations.  It  therefore  includes  psychic  activity  rang- 
ing from  simple  associative  memory  to  complex  trains  of 
reasoning.  What  distinguishes  intelligence  from  instinct  is 
that  in  the  latter  the  connections  between  acts  are  based 
upon  hereditary  organization,  whereas  in  the  former  they 
are  established  through  experience.  The  apparently  new 
thing  involved  in  intelligent  behavior  is  the  power  of  form- 
ing associations.  So  far  as  we  can  judge  of  the  psychic  states 
of  an  animal  from  its  behavior,  animal  intelligence  in  its  first 
manifestations  consists  in  repeating  acts  which  bring  pleasure 
and  in  avoiding  things  which  cause  pain,  and  a  discussion  of 
the  transition  from  instinct  to  intelligence  naturally  involves 

^  This  chapter  is  taken  with  some  modifications  from  an  article  of  the 
same  title  contributed  by  the  writer  to  the  Journal  of  Comparative  Neu- 
rology and  Psychology.  I  am  indebted  to  the  editor  for  permission  to 
reprint  a  considerable  part  of  the  article  here. 

164 


BEGINNINGS  OF  INTELLIGENCE  165 

a  consideration  of  the  role  of  pleasure  and  pain  as  agents  of 
accommodation. 

The  tendency  of  animals  to  repeat  acts  which  result  in 
pleasure  and  to  discontinue  or  inhibit  acts  which  bring  them 
pain  is  a  fundamental  feature  of  behavior  on  the  utility  of 
which  it  would  be  superfluous  to  comment.  But  why  do 
animals  behave  in  this  fortunate  manner,  and  how  did  they 
come  to  acquire  the  faculty  of  so  behaving?  To  our  ordinary 
plain  w^ay  of  thinking  it  appears  sufficient  to  say  that  a  dog 
eats  meat  because  he  likes  it,  and  that  he  runs  away  from  the 
whip  to  avoid  its  painful  incidence  upon  his  integument. 
These  acts  are  such  natural  and  obvious  things  to  do  under 
the  circumstances  that  to  inquire  why  the  animal  does  what 
it  likes  and  avoids  what  is  disagreeable  may  seem  a  sort 
of  philosophic  quibble  which  only  a  mind  "debauched  by 
learning"  would  think  of  indulging  in.  But  a  little  con- 
sideration will  show  that  we  have  here  a  real  and  very 
knotty  problem,  or  rather  set  of  problems,  of  the  greatest 
importance  to  the  student  of  genetic  psychology. 

There  are  few  better  illustrations  of  the  modification  of 
behavior  through  experiences  of  pleasure  and  pain  than  that 
afforded  by  the  behavior  of  young  chicks,  which  has  been 
so  well  studied  by  Lloyd  Morgan.  A  young  chick  when 
first  hatched  has  the  instinct  to  peck  at  all  sorts  of  objects 
of  about  a  certain  size.  If  an  object  is  a  little  too  large  the 
chick  may  hesitate.  Should  it  venture  to  peck  at  the  object 
and  derive  a  pleasant  taste  from  it  the  hesitation  in  the  pres- 
ence of  similar  objects  becomes  reduced  and  will  finally 
disappear.  If  the  chick  in  the  course  of  its  pecking  seizes 
a  caterpillar  having  a  nauseous  taste  it  is  much  less  apt  to 
seize  a  similar  caterpillar  a  second  time.  The  painful  or 
unpleasant  experience  it  derives  in  some  way  inhibits  further 
action  toward  that  class  of  objects. 


166  BEGINNINGS  OF  INTELLIGENCE 

We  have  in  this  modification  of  instincts  through  the 
pleasurable  or  painful  effects  they  produce  the  beginning  of 
intelligence.  The  pecking,  swallowing,  and  avoidance  of 
certain  objects  are  purely  instinctive  acts  based  on  the 
chick's  inherited  organization.  After  its  first  experiences 
with  pleasant  or  nasty  caterpillars  the  chick  is  a  different 
creature;  it  has  learned  by  experience;  and  henceforth  its 
acts,  which  at  first  were  in  a  general  way  adaptive,  become 
more  perfectly  adapted  to  its  needs  as  the  result  of  its  learn- 
ing. Instinct  supplied  the  impetus  to  action  and  in  a  measure 
determined  the  direction  of  action,  but  intelligence  refines 
upon  the  instinctive  behavior  and  effects  a  closer  adjustment 
to  the  environment. 

In  lower  forms  associations  are  formed  as  a  rule  with  great 
slowness.  Behavior  is  almost  entirely  instinctive,  and  the 
organism  can  be  made  to  deviate  from  its  stereotyped 
methods  of  action  only  with  difficulty.  It  is  probable  that 
in  low  forms  where  associations  of  only  the  simplest  kind  can 
be  established  there  is  no  association  of  ideas  involved; 
and  in  fact  there  is  no  conclusive  evidence  of  the  existence 
of  ideas  even  in  animals  quite  high  in  the  scale.  Most  animal 
learning  consists  in  forming  associations  between  certain 
sense  experiences  and  certain  actions  which  bring  pleasure 
or  pain.  A  common  way  of  teaching  an  animal  a  trick  is  to 
try  in  various  ways  to  induce  it  to  perform  the  desired 
action  and  then  to  reward  it  by  food  or  some  other  means 
of  giving  it  pleasure.  In  this  way  the  connection  between 
the  situation  and  the  act  is  reinforced,  and  the  act  follows 
more  readily  when  the  animal  is  placed  a  second  time  under 
the  same  conditions. 

Consider  the  case  of  a  cat  placed  in  a  box  which  can  be 
opened  by  pressing  down  a  lever  or  pulling  a  string,  as  in  the 
experiments  of  Thorndike.    If  the  cat  is  hungry  and  food 


BEGINNINGS  OF  INTELLIGENCE  167 

is  placed  outside,  the  animal  will  probably  make  vigorous 
efforts  to  escape  by  clawing  and  biting  in  various  parts  of 
the  enclosure,  which  are  the  usual  instinctive  methods  em- 
ployed in  similar  situations.  If  the  right  movement  is  hit 
upon  and  the  cat  gets  out  and  secures  food,  it  will  probably 
make  its  escape  more  readily  than  before  when  placed  in 
the  box  a  second  time.  After  a  number  of  trials  the  cat 
will  come  to  make  the  right  movements  for  escaping  very 
soon  after  being  placed  in  the  box  and  its  useless  random 
movements  will  be  discontinued.  The  connection  between 
the  perception  of  the  mechanism  of  escape  in  the  box  and 
the  act  necessary  to  gain  its  liberty  comes  to  be  more  and 
more  firmly  established  in  the  cat's  brain  with  repeated 
experiences.  The  cat  perceives  a  number  of  things  in  the 
box  and  performs  a  number  of  different  acts,  but  out  of 
all  these,  associations  are  formed  only  between  certain 
stimuli  and  those  responses  to  them  which  bring  pleasure 
to  the  animal. 

Pleasure  and  pain  have  apparently  a  fundamental  con- 
nection with  the  development  of  intelligent  responses  out 
of  instinctive  activity.  Were  there  not  something  to  clinch 
or  strengthen  the  connection  between  certain  stimuli  and 
the  appropriate  responses  to  them  the  organism  might  per- 
form random  movements  till  doomsday  without  being  a 
whit  better  off.  It  is  a  problem  therefore  of  fundamental 
importance  to  ascertain  in  what  the  mechanism  of  this 
ability  to  profit  by  experience  essentially  consists.  It  is 
not  mere  habit,  not  the  mere  making  more  permeable  certain 
preformed  connections  in  the  brain.  One  act  would  then  be 
just  as  apt  to  be  followed  up  as  another.  Whether  an  act 
tends  to  be  followed  or  not  depends  on  what  it  brings  to  the 
organism.    Apparently  we  have  to  do  with  a  selective 


168  BEGINNINGS  OF  INTELLIGENCE 

agency  which  preserves  or  repeats  certain  activities  and  re- 
jects others  on  the  basis  of  their  results. 

The  importance  of  random  movements  lies  in  the  fact  that 
they  offer  opportunities  for  making  favorable  adjustments. 
For  the  development  of  intelligence  they  play  a  role  similar 
to  that  of  variations  in  the  process  of  evolution.  The 
animal  that  does  the  most  exploration  is  the  one  most  likely  to 
hit  upon  new  advantageous  adjustments.  In  the  same  way 
intelligent  adjustments,  as  James  has  contended,  are  favored 
by  a  multiplicity  of  instincts,  especially  if  these  instincts  are 
of  a  contrary  or  conflicting  nature,  for  now  one  and  now 
another  instinctive  tendency  may  be  reinforced  in  different 
conditions  to  which  it  may  be  adapted.  Instinctive  fear 
may  be  modified  through  experience  so  that  it  is  no  longer 
attached  to  objects  that  are  found  to  be  harmless,  while  it  may 
be  intensified  in  relation  to  other  objects  that  are  found 
to  be  sources  of  injury.  Where  there  is  hesitation  between 
the  exercise  of  two  instincts  such  as  the  tendency  to  pursue 
an  animal  as  prey,  and  the  instinctive  fear  which  that  animal 
may  awaken,  experience  may  quickly  point  out  which 
proclivity  is  the  more  advantageous  to  follow.  The  pleasure- 
pain  reaction  enables  an  animal  to  select,  so  to  speak  out  of 
its  stock  of  instinctive  endowments  those  responses  which 
are  best  adapted  to  the  particular  situations  that  confront 
it.  It  is  a  means  of  adapting  instincts  to  new  or  inconstant 
conditions  and  thus  of  effecting  a  closer  adaptation  to  the 
environment  than  that  which  would  be  possible  by  following 
purely  congenital  modes  of  response.  The  development  of 
the  pleasure-pain  reaction  marks  one  of  the  most  important 
steps  in  the  evolution  of  behavior,  for  the  entire  super- 
structure of  intelligence  in  all  its  stages  is  based  upon  it, 
and  it  is  not  surprising  that  many  writers  regard  it  as  an 
index  of  the  beginning  of  consciousness,  a  point  where  a  new 


BEGINNINGS  OF  INTELLIGENCE  169 

entity  is  somehow  mysteriously  injected  into  the  miiverse. 

It  is  a  general  rule  that  what  is  pleasant  is  beneficial  and 
what  is  painful  is  injurious,  and,  therefore,  by  following 
its  desires  and  aversions  an  animal  is  guided  in  a  tolerably 
safe  course.  Eating  when  hungry,  drinking  when  thirsty, 
seeking  warmth  when  cold,  exercise  when  in  a  state  of 
vigor,  and  rest  when  fatigued,  all  bring  a  state  of  satisfac- 
tion or  pleasure.  On  the  other  hand,  eating  and  drinking 
after  a  certain  stage  of  repletion  has  been  reached,  or  at- 
taining too  great  a  degree  of  warmth  may  be  positively 
painful,  the  pain  being  correlated  with  carr)dng  on  these 
activities   until   they   become  injurious  to  the  organism. 

But  it  is  well  known  that  this  correlation  is  not  an  absolute 
one.  With  complex  creatures  like  ourselves  with  a  multitude 
of  different  propensities  and  interests  it  is  not  infrequent 
that  the  pursuit  of  what  is  agreeable  leads  to  all  sorts  of 
unfortunate  consequences,  even  of  a  purely  physiological 
nature.  In  the  lower  animals  where  pleasure  is  a  safer 
guide  than  among  ourselves,  what  is  pleasant  is  not  always 
what  is  organically  good.  Poisonous  articles  may  be  eaten 
with  apparent  relish  and  alcoholic  liquors  are  readily  im- 
bibed even  by  such  primitive  creatures  as  bees  and  wasps 
upon  their  very  first  acquaintance  with  these  intoxicants. 
But  aside  from  exceptional  cases,  pleasure  in  the  animal 
world  is  a  sufficiently  good  index  of  what  is  beneficial  that 
under  conditions  which  ordinarily  present  themselves  it 
seldom  leads  to  injurious  courses  of  action. 

The  relation  between  the  pleasant  and  the  beneficial  is, 
however,  probably  not  a  primary  one,  and  it  is  not  improb- 
able that  it  represents  a  connection  established  by  natural 
selection,   as   was  first   maintained   by   Herbert   Spencer. 

"  If  the  states  of  consciousness  which  a  creature  endeavors 
to  maintain  are  the  correlatives  of  injiuious  actions  and  if 


170  BEGINNINGS  OF  INTELLIGENCE 

the  states  of  consciousness  which  it  endeavors  to  expel  are 
the  correlatives  of  beneficial  actions^  it  must  quickly  disap- 
pear through  persistence  in*  the  injurious  and  avoidance 
of  the  beneficial.  In  other  words,  those  races  of  beings  only 
can  have  survived  in  which,  on  the  average,  agreeable  or 
desired  feelings  went  along  with  activities  conducive  to  the 
maintenance  of  life,  while  disagreeable  and  habitually- 
avoided  feelings  went  along  with  activities  directly  or  in- 
directly destructive  of  life;  and  there  must  ever  have  been, 
other  things  equal,  the  most  useful  and  long-continued 
survivals  among  races  in  which  these  adjustments  of  feelings 
to  actions  were  the  best,  tending  ever  to  bring  about  perfect 
adjustment/' 

This  explanation  which  has  become  widely  accepted 
leaves  a  fundamental  question  unanswered.  It  does  not 
explain  why  certain  acts  are  stamped  in  and  certain  others 
stamped  out.  Of  the  mechanism  of  this  process,  which  is 
the  real  problem  involved  in  the  pleasure-pain  reaction, 
we  are  as  ignorant  as  before.  The  explanation  means  that 
animals  which  took  pleasure  in  following  acts  that  brought 
them  benefit  were  preserved  and  those  that  did  not  behave  in 
this  manner  were  eliminated.  But  why  does  an  animal  tend 
to  repeat  an  act  that  brings  it  pleasure  and  avoid  one  that 
produces  pain?  It  seems  so  natural  for  creatures  to  behave 
in  this  way  that  the  existence  of  any  problem  here  is  usually 
unsuspected,  but  this  is  the  problem  that  confronts  us  when 
we  endeavor  to  obtain  a  clear  understanding  of  the  way 
in  which  intelligence  develops  out  of  instinct. 

In  the  pleasure-pain  response  we  have  two  problems  of  a 
quite  different  nature:  (1)  the  problem  of  how  behavior  is 
modified  by  its  results,  and  (2)  the  problem  of  why  pleasure 
is  associated  with  certain  physiological  activities,  such  as 
securing  movements,  and  pain  with  others,  such  as  avoiding 


BEGINNINGS  OF  INTELLIGENCE  171 

movements.  The  latter  problem  is  one  whose  solution 
appears  hopeless.  If  we  accept  the  doctrine  of  psycho- 
physical parallelism  in  any  of  its  forms,  we  must  deny  that 
psychic  states  are,  strictly  speaking,  the  causes  of  physical 
changes.  Why  then  should  pleasure  be  connected  with  one 
kind  of  activity  and  pain  with  another?  Why  not  just  the 
reverse?  This  problem  is,  I  believe,  insoluble,  because  it  is 
a  question  of  the  relation  of  the  physical  and  the  psychical; 
it  is  of  essentially  the  same  nature  as  the  question  why  one 
kind  of  retinal  stimulation  produces  a  sensation  of  red  and 
another  a  sensation  of  green.  Physical  and  psychical  states 
are  correlated  in  particular  ways;  this  we  accept  as  a  matter 
of  observed  connection.  But  why  a  certain  kind  of  brain 
vibration  is  associated  with  a  state  of  consciousness  we  call 
a  sensation  of  red  instead  of  some  other  state  is  a  question 
upon  which  we  may  intend  our  minds  indefinitely  without 
the  least  profit.  If  w^e  adopt  any  other  theory  of  the  relation 
of  mind  and  body  we  are  in  no  way  better  off.  If  we  have 
to  do  with  a  preordained  connection  of  pleasure  with  certain 
physiological  activities  and  pain  with  certain  others,  this 
connection  is  no  more  intelligible  if  we  admit  the  interaction 
of  psychical  and  physical  states  than  it  is  under  the  theory 
of  parallelism.  We  can  only  say  that  such  is  the  observed 
relation  of  the  phenomena. 

What  is  feasible  to  attempt  to  solve  is  the  problem  of  the 
adaptive  modification  of  behavior  which  we  may  say  is  the 
objective  side  of  the  pleasure- pain  process.  We  are  dealing 
with  a  series  of  physiological  reactions  and  how  they  come 
to  be  modified.  We  may  assume  that  psychical  states  enter 
into  the  chain  of  causes  and  effects  that  make  up  an  animal's 
behavior,  but  it  is  not  clear  that  such  an  assumption  throws 
the  least  light  upon  our  problem,  and  it  is  open  to  serious 
objections  on  both  scientific  and  metaphysical  grounds.    We 


172  BEGINNINGS  OF  INTELLIGENCE 

shall  therefore  consider  the  question  purely  from  a  phy- 
siological standpoint.  Viewed  objectively  we  find  that 
in  an  animal's  behavior  certain  acts  when  once  performed 
tend  to  be  performed  with  greater  readiness  under  similar 
conditions  a  second  time,  while  other  acts  once  performed 
tend  under  similar  conditions  to  be  inhibited.  This  prob- 
lem of  learning,  Baldwin  observes  "is  the  most  urgent, 
difficult  and  neglected  question  in  the  new  genetic  psychol- 
ogy." Spencer  with  his  characteristic  insight  into  funda- 
mental problems  has  grappled  with  it  and  has  attempted 
to  give  a  physiological  explanation.  Pleasure,  according 
to  Spencer,  is  the  concomitant  of  heightened  nervous  dis- 
charge; pain  the  concomitant  of  lessened  discharge.  In  an 
animal  with  a  diffuse  discharge  of  its  nervous  energy  result- 
ing in  random  movements,  some  of  these  movements  bring 
a  heightened  nervous  discharge  with  its  psychic  accompani- 
ment of  pleasure.  This  tends  to  reinforce  the  movement  that 
brought  the  increase  of  nervous  energy  and  to  cause  it  to  be 
repeated.  Responses  resulting  in  pain  tend  on  account  of  the 
diminution  of  nervous  discharge  that  follows  to  be  discon- 
tinued, and  in  this  way  the  organism  is  kept  repeating  certain 
acts  and  avoiding  others.  "Along  with  the  concentrated 
discharge  to  particular  muscles,''  says  Spencer,  "the  gang- 
lionic plexuses  inevitably  carry  off  a  certain  diffused  dis- 
charge to  the  muscles  at  large,  and  this  diffused  discharge 
produces  on  them  very  variable  results.  Suppose,  now, 
that  in  putting  out  its  head  to  seize  prey  scarcely  within 
reach,  a  creature  has  repeatedly  failed.  Suppose  that  along 
with  the  group  of  motor  actions  approximately  adapted  to 
seize  prey  at  this  distance,  the  diffused  discharge  is,  on 
some  occasion,  so  distributed  throughout  the  muscular 
system  as  to  cause  a  slight  forward  movement  of  the  body. 
Success  will  occur  instead  of  failure;  and  after  success  will 


BEGINNINGS  OF  INTELLIGENCE  173 

immediately  come  certain  pleasurable  sensations  with  an 
accompanying  draught  of  nervous  energy  toward  the 
organs  employed  in  eating,  etc.  That  is  to  say,  the  lines 
of  nervous  communication  through  which  the  diffused  dis- 
charge happened  in  this  case  to  pass,  have  opened  a  new 
way  to  certain  wide  channels  of  escape;  and,  consequently, 
they  have  suddenly  become  lines  through  which  a  large 
quantity  of  molecular  movements  that  were  followed  by 
success  are  likely  to  be  repeated;  what  was  at  first  an  acci- 
dental combination  of  motions  will  now  be  a  combination 
having  considerable  probability/' 

Bain's  view  of  learning  is  much  like  that  of  Spencer. 
'^We  suppose  movements  spontaneously  begun,  and  acci- 
dentally causing  pleasure;  we  then  assume  that  with  the 
pleasure  there  will  be  an  increase  of  vital  energy,  in  which 
increase  the  fortunate  movements  will  share,  and  thereby 
increase  the  pleasure.  Or,  on  the  other  hand,  we  suppose 
the  spontaneous  movements  to  give  pain,  and  assume  that, 
with  the  pain,  there  will  be  a  decrease  of  energy,  extending 
to  the  movements  that  cause  the  evil,  and  thereby  providing 
a  remedy.  A  few  repetitions  of  the  fortuitous  concurrence 
of  pleasure  and  a  certain  movement,  will  lead  to  the  forging 
of  an  acquired  connection,  under  the  law  of  retentiveness 
or  contiguity,  so  that,  at  an  after  time,  the  pleasure  or  its 
idea  shall  evoke  the  proper  movement  at  once." 

The  theories  of  Bain  and  Spencer  are  (Jiscussed  in  de- 
tail by  Baldwin,  who,  while  differing  from  these  writers  in 
certain  points  which  need  not  here  be  dwelt  upon,  adopts 
essentially  the  same  view  as  regards  the  mechanism  of  re- 
inforcement and  inhibition.  With  Bain  and  Spencer, 
Baldwin  assumes  that  "  the  pleasure  resulting  from  the  first 
accidentally  adaptive  movement,  issues  in  a  heightened 
nervous  discharge  toward  the  organs  which  made  the  move- 


174  BEGINNINGS  OF  INTELLIGENCE 

ment,  a  discharge  which  finds  its  way  to  the  same  channels 
as  before,  and  so  makes  it  likely  that  the  same  movement 
will  be  repeated,  the  external  conditions  remaining  the  same. 
Pleasure  and  pain  can  be  agents  of  accommo- 
dation and  development  only  if  the  one  pleasure,  carry 
with  it  the  phenomenon  of  'motor  excess,'  and  the  other, 
pain,  the  reverse — probably  some  form  of  inhibition  or  of 
antagonistic  contraction." 

The  theories  of  Spencer,  Bain  and  Baldwin  are  physio- 
logical since  they  attempt  to  explain  the  modifications  of 
behavior,  not  through  the  influence  of  certain  psychic  states, 
but  as  the  effect  of  the  physiological  conditions  of  which  these 
states  are  the  concomitants.  The  theories  are  all  open  to  the 
objection  that  pleasure  is  by  no  means  the  constant  con- 
comitant of  heightened  nervous  discharge.  Laughing  and 
crying  are  very  similar  in  their  physiological  expression, 
though  they  go  along  with  very  different  psychic  states, 
A  child  who  burns  his  hands  and  writhes  about  in  agony 
certainly  manifests  a  heightened  nervous  discharge,  but  he 
shows  no  tendency  to  put  his  hands  again  into  the  fire. 
Another  outreaching  movement  of  the  child  brings  his  hands 
toward  a  pleasant  degree  of  warmth.  The  movement 
tends  to  be  repeated.  The  nervous  discharge  in  the  first 
case  is  much  greater  than  in  the  second,  but  in  both  cases 
it  goes  to  the  arm,  though  along  somewhat  different  nerves. 
It  is  obvious,  I  think,  that  we  cannot  account  for  the  differ- 
ence between  the  responses  to  pleasurable  and  painful 
stimuli  on  the  basis  of  any  quantitative  difference  in  the 
discharges  to  the  part  affected.  It  is  a  matter  of  nervous 
connection  rather  than  quantity  of  nervous  energy. 

Pain-giving  stimuli,  owing  to  the  arrangement  of  an  animal's 
reflex  arcs,  are  generally  followed  by  a  withdrawing  move- 
ment of  the  part  stimulated,  but  that  there  is  a  tendency 


BEGINNINGS  OF  INTELLIGENCE  175 

for  the  "increased  energy  of  the  pleasure  process"  to  flow 
"into  the  channels  of  the  movement  associated  with  pleasm-e" 
(that  iS;  I  take  it,  the  movement  which  brings  pleasm-e) 
is  by  no  means  evident.  There  is,  I  think,  no  primary  ten- 
dency, as  Spencer  and  Bain  seem  to  think,  for  the  nervous 
discharge  to  take  the  direction  of  the  organ  from  which  the 
pleasure  is  derived.  Animals,  it  is  true,  move  so  as  to 
bring  an  organ  which  is  pleasantly  stimulated  again  under 
the  action  of  the  stimulus,  but  this  is  often  due  to  the  dis- 
charge going  mainly  to  a  quite  different  part  of  the  body, 
such  as  distant  appendages,  instead  of  to  the  part  directly 
affected. 

The  theory  of  heightened  nervous  discharge  as  expounded 
by  Spencer,  Bain  and  Baldwin,  fails  to  give  us,  I  think,  the 
desired  explanation  of  the  acquirement  of  individual  ac- 
commodations, and  one  naturally  turns  to  other  theories  of 
the  psycho-physiology  of  pleasure  and  pain  for  light.  Here, 
however,  we  are  led  into  a  veritable  quagmire  of  psychological 
speculation,  for  there  are  few  fields  in  which  there  are  so 
many  and  so  fundamental  differences  of  opinion  among 
competent  psychologists.  The  physiological  concomitants 
of  pleasure  and  pain  have  afforded  a  subject  of  numerous 
laboratory  studies  and  almost  no  end  of  theories.  There 
is  good  evidence  that  pain  sensations  are  produced  by 
the  stimulation  of  specific  nerves,  but  as  regards  the 
physiological  states  accompan3dng  pleasure  and  pain  the 
results  of  experiments  as  well  as  opinions  based  on  them 
are  very  discordant.  And  it  is  difficult  to  see  how  most 
of  the  pleasure-pain  theories  would  help  us  to  explain 
the  mechanism  of  accommodation  even  if  they  were 
established,  so  that  the  outlook  for  the  solution  of  the 
problem  as  it  is  commonly  formulated  does  not  seem,  at 
present,  an  encouraging  one. 


176  BEGINNINGS  OF  INTELLIGENCE 

A  new  point  of  view  in  regard  to  our  problem  has  been 
presented  by  Hobhouse  in  his  Mind  in  Evolution.  To 
illustrate  this  view  let  us  recur  to  our  chick.  When  a  nasty 
caterpillar  is  seen  for  the  first  time  the  visual  stimulus  sets 
up  a  pecking  reaction.  This  is  followed  by  the  stimulus  of  a 
bad  taste  which  sets  up  various  rejection  movements,  such 
as  ejection  of  the  food  and  wiping  the  bill.  The  order  of 
events  is: 

Stimulus.  .  .  .  pecking.  .  .  .  bad  taste.   .  .  .rejection. 

When  the  same  kind  of  caterpillar  is  met  with  a  second 
time  the  stimulus  tends  to  elicit  the  rejection  movements 
with  which  it  has  been  associated  instead  of  the  movements 
of  pecking.  Is  not  the  inhibition  due  to  the  fact  that  the 
stimulus  has  become  associated  with  a  response  which  is 
incongruous  with  the  first?  Movements  of  rejection  and 
avoidance  are  incompatible  with  those  of  pecking  and  swal- 
lowing and  it  may  therefore  be  unnecessary  to  look  to  any 
peculiarity  of  the  physiological  correlates  of  pain  for  an 
explanation  of  the  inhibition  of  the  original  reaction.  The 
stimulus  becomes  coupled  with  a  new  reflex  arc;  nervous  en- 
ergy is  drained  off  in  a  new  channel,  and  the  future  behavior 
becomes  changed.  If  the  taste  is  a  very  bad  one,  a  great  deal 
of  energy  is  involved  and  the  connection  with  the  rejec- 
tion response  made  very  permeable  and  the  rejection  move- 
ment easily  set  up.  If  a  person  is  confronted  with  a  sight 
of  some  nauseating  medicine  he  has  recently  taken,  avoid- 
ing or  rejection  movements  are  set  up,  such  as  making  a 
face,  or  even  retching  movements  of  the  stomach.  Is  it 
not  these  movements  or  attempts  at  movements  that  really 
inhibit  the  taking  of  the  medicine?  This  is  evinced  by  the 
chick  described  by  Lloyd  Morgan,  which  after  an  experience 
with  a  nasty  caterpillar  approached  one  a  second  time, 
but  stopped  and  wiped  its  bill  and  went  away  as  if  it  actually 


BEGINNINGS  OF  INTELLIGENCE  177 

repeated  its  first  experience.  Of  coulee  inhibition  of  the 
original  response  does  not  always  involve  contrary  move- 
ments, but  there  may  be  impulses  to  such  movements  which 
do  not  issue  in  action.  The  principal  feature  in  the  modi- 
fication of  action  through  painful  experiences  is  the  assimi- 
lation of  impulses  incongruous  with  the  original  one. 

In  the  reinforcement  or  stamping  in  of  a  reaction  to  a 
particular  stimulus  that  brings  pleasure,  it  certainly  seems 
as  if  pleasure  or  its  physiological  correlate  in  some  way 
serves  to  cement  more  firmly  the  association  between  the 
stimulus  and  the  response.  Let  us  consider,  however, 
the  case  in  which  the  chick  pecks  at  a  caterpillar  which 
has  a  good  taste.  The  presence  of  the  caterpillar  in  the 
mouth  excites  the  swallowing  reflexes;  in  the  presence  of  a 
similar  caterpillar  the  pecking  response  is  made  more  readily 
than  before,  and  whatever  hesitation  there  may  have  been  at 
first  disappears.  Is  not  the  difference  from  the  pain-re- 
sponse due  to  the  fact  that  there  is  an  organic  incompatibOity 
between  the  first  and  second  responses  in  the  pain  response, 
while  there  is  an  organic  congruity  or  mutual  reinforcement 
of  these  responses  in  the  other?  Pecking  and  swallowing 
form  the  normal  elements  of  a  chain  reflex;  when  one  part 
of  the  system  is  excited  it  tends  to  excite  the  rest,  to  increase 
the  general  tonus  of  all  parts  concerned  in  the  reaction. 

According  to  the  view  here  presented,  whether  a  particular 
response  to  a  stimulus  tends  to  be  repeated  more  readily 
or  discontinued,  depends  not  upon  the  peculiar  physiological 
state  which  may  be  produced  in  the  brain,  but  upon  the 
kind  of  responses  which  the  stimuli  brought  by  the  act  call 
forth.  If  an  outreaching  reaction  becomes  coupled  with  a 
withdrawing  response  the  result  is  inhibition.  If  the  re- 
action, on  the  other  hand,  brings  stimuli  which  produce 
congruent   reactions   the   association   formed   with   these 

12 


178  BEGINNINGS  OF  INTELLIGENCE 

latter  reinforces  the  first  reaction.  The  pleasure-pain 
response  then  resolves  itself  into  the  formation  of  associa- 
tions. Withdrawing  and  defensive  responses  are  usually 
initiated  by  pain  giving  stimuli,  and  the  instinctive  or 
random  movement  which  brings  a  painful  stimulus  is  in- 
hibited under  similar  conditions  in  the  future,  not  because 
of  the  pain  of  its  physiological  correlate,  but  because  it 
comes  to  be  associated  with  a  withdrawing  or  defensive, 
and  hence  an  incongruous  or  inhibitory  reaction.  Pleasure 
and  pain  thus  interpreted  have  no  mysterious  power  of 
stamping  in  or  stamping  out  certain  associations.  Whether 
the  result  is  reinforcement  or  inhibition  depends  on  the 
way  in  which  a  reaction  and  the  secondary  responses  re- 
sulting from  the  situation  in  which  the  organism  is  thereby 
brought,  happen  to  harmonize. 

The  step  from  instinct  to  intelligence  viewed  as  a  physio- 
logical process  involves,  therefore,  no  essentially  new 
element  beyond  the  well  known  physiological  properties  of 
the  nervous  system,  and  we  are  not  committed  to  any  par- 
ticular hypothesis  as  to  the  physiological  accompaniments 
of  pleasure  and  pain,  or  pleasantness  and  unpleasantness, 
in  order  to  understand  how  behavior  may  become  adaptively 
modified.  How  far  the  interpretation  given  will  enable  us 
to  explain  the  development  of  intelligence  I  do  not  pretend 
to  say.  It  may  break  down  in  attempts  to  apply  it  to 
higher  forms  of  learning,  but  it  affords  a  useful  working 
hypothesis  and  takes  us  a  way,  I  think,  toward  the  solution 
of  our  problem. 

BIBLIOGRAPHY 
Bain,  A.    The  Sensations  and  the  Intellect,  3d.  ed.,  *94. 

The  Emotions  and  the  Will,  4th.  ed.,  '99. 
Baldwin,  J.  M.     Mental  Development  in  the  Child  and  in  the  Race. 

Methods  and  Processes,  2nd.  ed.,  N.  Y.,     '97. 

Development  and  Evolution,  N.  Y.,  '02. 


BEGINNINGS  OF  INTELLIGENCE  179 

HoBHOUSE,  L.  T.     Mind  in  Evolution,  '01. 

Holmes,  IS.  J.    Pleasure,  Pain  and  the  Beginnings  of  Intelligence. 

Jour.  Comp.  Neur.  Psych.,  20,  145,  '10. 

The  Beginnings  of  Intelligence,  Science,  33,  473,  '11. 
Marshall,  H.  R.    Pleasure,  Pain  and  -Esthetics,  N.  Y.,  '94, 
Spencer,  H.    Principles  of  Psychology,  1855. 


CHAPTER  IX 

PRIMITIVE  TYPES  OF  INTELLIGENCE  IN 
CRUSTACEANS  AND  MOLLUSCS 

It  is  scarcely  possible  at  present  to  fix,  even  with  the 
rudest  approximation,  the  point  where  intelligence  makes 
its  first  appearance  in  the  course  of  evolution.  There  is 
little  doubt  that  the  step  from  instinct  to  intelligence  has 
been  made,  not  once  merely,  but  several  times.  The  in- 
telligence of  the  higher  Mollusca  had,  in  all  probability,  an 
origin  independent  from  that  of  the  arthropods,  and  the  in- 
telligence of  the  vertebrates  was  probably  developed  in- 
dependently of  that  of  the  other  groups.  Among  the  arthro- 
pods themselves  it  is  not  likely  that  the  intelligence  manifested 
by  the  arachnids  had  a  common  origin  with  that  of  the 
insects,  and  within  both  of  these  large  groups  intelligence 
may  have  been  independently  developed  out  of  behavior  of 
the  purely  instinctive  type. 

Intelligence  grows  out  of  the  complexity  and  perfection 
of  the  nervous  mechanism,  and  along  whatever  line  organiza- 
tion reaches  a  certain  degree  of  development  intelligence 
appears  on  the  scene.  From  what  has  been  said  in  previous 
pages  we  are  prepared  to  appreciate  the  fact  that  intelligence 
is  not  an  entirely  new  power  unrelated  to  the  other  activities 
of  organic  life,  but  a  process  growing  out  of  other  organic 
functions  and  having  the  same  end  as  these  other  functions; 
it  is,  as  Spencer  has  so  well  emphasized,  but  a  higher  phase 
of  those  processes  of  adjustment  and  regulation  which  make 
up  the  life  of  the  animal. 

180 


PRIMITIVE  TYPES  OF  INTELLIGENCE         181 

The  criterion  of  intelligence  which  we  have  adopted — the 
power  of  forming  associations — is  one  which  is  accepted  by 
a  considerable  number  of  comparative  psychologists.  Un- 
fortunately the  term  intelligence  is  used  in  a  variety  of  senses 
by  different  vsTiters — a  fact  which  is  in  part  responsible  for 
the  different  expressions  of  opinion  as  to  where  in  the  animal 
kingdom  intelligence  makes  its  beginning.  The  acute 
Father  Wasmann  wdll  have  none  of  intelligence  in  any 
animal  below  man,  but  as  he  defines  it,  the  term  connotes 
the  power  of  reasoning  by  the  use  of  general  concepts. 
The  controversy  which  has  arisen  over  this  employment  of 
the  term  is  a  matter  for  the  lexicographer  instead  of  the 
psychologist,  and  so  long  as  a  WTiter  makes  his  peace  with 
the  dictionary  we  have  no  quarrel  with  him.  We  prefer, 
however,  to  employ  the  term  in  its  more  widely  accepted 
meaning. 

As  stated  in  a  previous  chapter,  there  is  no  evidence  that 
there  is  any  power  of  forming  associations  in  the  Protozoa. 
In  the  Coelenterata  behavior,  although  of  the  reflex  type, 
is  often  higlily  plastic  and  capable  of  being  modified  in 
many  ways  as  the  result  of  previous  experience;  but  while 
intelligence  has  often  been  claimed  for  these  forms,  there  is, 
in  the  opinion  of  the  wTiter,  no  case  in  which  the  formation 
of  associations  is  satisfactorily  proven.  The  same  state- 
ment may  also  be  risked  for  that  large  and  miscellaneous 
assortment  of  animals  grouped  under  the  term  Vermes. 
The  behavior  of  Echinoderms  is  certainly  complex  and  plastic 
to  a  remarkable  degree,  but  even  in  this  group  the  power 
of  forming  associations  is  very  doubtful.  Preyer,  who  has 
made  a  very  thorough  study  of  the  behavior  of  the  starfish, 
claims  to  have  discovered  indubitable  indications  of  intelli- 
gent action,  but  the  later  studies  of  Jennings  andGlaseronthe 
behavior  of  starfish  and  ophiurans  failed  to  confirm  Preyer's 


182        PRIMITIVE  TYPES  OF  INTELLIGENCE 

conclusions.  A  starfish  may  possibly  acquire  habits  of  a 
certain  kind,  but  it  is  not  proven  that  it  is  able  to  form 
associations. 

We  do  not  intend  to  deny  the  existence  of  intelligence  in 
the  groups  mentioned;  we  think  it  not  improbable  that 
intelligence  of  a  primitive  sort  may  be  discovered,  at  least 
in  the  more  highly  developed  members  of  these  divisions; 
but  at  the  present  time  we  can  only  grant  the  Scotch  verdict 
of  "not  proven." 

In  the  Arthropoda  instinctive  activity  is  frequently  re- 
presented as  reaching  its  culmination,  and  some  investigators 
have  gone  so  far  as  to  assert  that  the  behavior  of  these  animals 
is  made  up  entirely  of  instincts  and  reflexes.  This  opinion 
is  in  part  based  on  a  priori  deductions  from  the  organization 
of  the  nervous  system  and  it  is  held  to  chiefly  by  morpholo- 
gists  and  physiologists  whose  observation  of  the  behavior  of 
animals  is  limited  and  warped  by  preconceptions. 

Bethe,  who  has  done  a  large  amount  of  thorough  and 
valuable  work  on  the  anatomy  and  physiology  of  the  nervous 
system  of  arthropods,  and  who  has  very  successfully  employed 
the  results  of  these  investigations  in  the  analysis  of  normal 
behavior,  was  led  to  the  somewhat  extreme  position  of 
denying,  not  only  associative  memory,  but  consciousness 
as  well,  in  all  the  arthropods.  The  complex  behavior  of 
these  forms,  according  to  him,  can  be  analyzed  in  terms  of 
reflex  action,  and  there  is  consequently  no  ground  for  assum- 
ing any  psychic  elements  whatsoever  in  these  animals. 

At  the  close  of  his  important  memoir  on  the  nervous 
system  of  the  crab  Carcinus  mcenas,  there  are  described  a 
few  experiments  which  convinced  Bethe  that  this  animal 
is  unable  to  profit  by  experience.  Bethe  placed  a  crab  in  an 
aquarium  containing  a  devil  fish,  Eledone,  which  took  up 
its  station  in  a  dark  corner.    The  crab  when  placed  in  the 


PRIMITIVE  TYPES  OF  INTELLIGENCE         183 

aquarium  quickly  rushed,  in  obedience  to  its  proclivity  to 
shun  the  light,  into  the  dark  comer  where  it  got  into  difficul- 
ties with  the  de\dl  fish.  Bethe  now  interfered  and  freed  the 
crab  from  its  captor  and  put  it  back  in  the  aquarium.  Back 
the  crab  went  again  into  the  arms  of  the  enemy.  Five 
successive  times  it  repeated  the  performance  (and  another 
individual  did  the  same  six  times)  without  learning  to  avoid 
the  retreat  of  the  devil  fish. 

In  another  experiment  a  piece  of  meat  was  placed  in  an 
aquarium  containing  some  hungry  crabs,  and  the  hand  of 
the  experimenter  was  held  over  the  meat.  ^Mienever  a 
crab  seized  the  food  the  creature  was  maltreated  and  driven 
away;  it  was  thought  that  if  the  crab  were  capable  of  learning 
it  would  come  to  associate  the  sight  of  the  experimenter's 
hand  with  the  painful  experience  following  the  seizure  of 
the  meat  and  keep  at  a  distance.  After  several  such  ex- 
periences it  went  after  the  meat  as  at  first,  and  Bethe 
concluded  that  the  creatures  were  nothing  but  "reflex 
machines,"  without  a  glimmer  of  intelligence. 

These  few  experiments  by  which  the  intelligence  of  the 
crab  is  summarily  disposed  of,  form  an  almost  amusing 
contrast  to  the  long,  detailed  and  exhaustive  work  on  the 
anatomy  and  physiology  of  the  nervous  system.  The 
experiments  are  obviously  inadequate,  not  only  because 
they  are  much  too  few  in  number,  but  because  they  do  not 
afford  the  best  opportunities  for  bringing  out  whatever 
power  of  forming  associations  a  crab  may  possess.  In  the 
first  experiment,  granting  that  the  crabs  were  not  more 
afraid  of  Bethe  than  of  the  devil  fish,  as  they  had  apparently 
as  much  reason  for  being,  it  would  have  been  necessary  for 
the  crab  to  inhibit  a  strong  instinct  before  it  could  manifest 
any  tendency  it  may  have  acquired  to  avoid  the  dark  comer 
with  its  sinister  occupant.     A  crab  when  afraid  makes  for  a 


184        PRIMITIVE  TYPES  OF  INTELLIGENCE 

dark  hole  if  there  is  any  within  reach;  and  with  its  whole 
energies  bent  upon  getting  away  from  the  large  creature 
into  whose  hands  it  is  taken,  what  wonder  if  the  devil  fish, 
if  it  would  otherwise  be  remembered,  should  be  temporarily 
forgotten. 

The  second  experiment  likewise  is  one  which  involves  the 
conquest  of  a  strong  instinctive  proclivity  and  it  includes 
also  too  small  a  number  of  trials  to  be  in  any  way  con- 
vincing. 

The  later  experiments  of  Yerkes  on  Carcinus  were  more 
fortunate  in  yielding  positive  results.  The  crabs  were  placed 
in  a  box  one  end  of  which  led  to  an  aquarium.  The  end 
nearest  the  aquarium  was  divided  so  as  to  afford  a  right  and 
a  wrong  path  to  the  water.  With  successive  trials  the  crab 
came  to  learn,  although  with  extreme  slowness,  to  choose 
the  right  path.  Other  simple  labyrinths  were  employed 
and  the  crab  in  each  case  succeeded,  after  a  sufficient  number 
of  trials,  in  learning  the  way  to  the  water.  The  work  of 
Cowles  on  Ocypoda  yielded  confirmatory  results,  although 
the  idiosyncrasies  of  the  animal  caused  the  results  to  be 
somewhat  less  clearly  defined. 

Experiments  similar  to  those  on  Carcinus  were  performed 
by  Yerkes  and  Huggins  on  the  crayfish.  A  simple  labyrinth 
was  constructed  consisting  of  a  box  having  a  small  compart- 
ment at  one  end,  and  an  opening  at  the  other  leading  to  an 
aquarium.  From  the  open  end  a  median  partition  extended 
back  a  short  distance,  and  one  of  the  passages  so  formed 
was  closed  with  a  glass  plate.  The  crayfish  liberated  from 
the  small  compartment  was  provided  with  a  choice  of  two 
paths  only  one  of  which  would  lead  it  to  the  water;  and  the 
endeavor  was  made  to  ascertain  if  the  crayfish,  after  a  number 
of  trials,  would  unerringly  choose  the  right  path.  The 
crayfish  used  were  put  through  a  number  of  preliminary  ex- 


PRIMITIVE  TYPES  OF  INTELLIGENCE         185 

periments  with  both  passages  open  to  determine  if  they  had 
any  tendency  to  go  toward  the  right  or  the  left,  and  after 
it  was  shown  that  either  path  was  chosen  with  equal  readi- 
ness, the  glass  plate  was  inserted  and  the  animals  put  again 
into  the  box.  In  the  first  experiment  the  crayfish  took  the 
correct  path  in  50  per  cent,  of  the  trials,  and  during  the 
subsequent  trials  the  percentage  of  correct  choices  gradually 
rose  until  in  the  final  ten  trials  it  reached  90  per  cent.  The 
improvement  was  very  gradual,  as  is  indicated  by  the  follow- 
ing series  of  percentages  of  successful  trials  for  each  set  of 
ten  trips:  50,  60,  75.8,  83.3,  76.6,  90.  Although  slowly 
acquired,  the  habit  of  following  the  right  path  was  not 
forgotten  after  an  interval  of  two  weeks. 

Further  experiments  in  which  the  box  was  thoroughly 
washed  out  after  each  trip  to  eliminate  any  guiding  influence 
of  odor  or  moisture  gave  similar  results.  If  after  a  cray- 
fish had  learned  to  turn  to  the  right,  the  right  pathway  was 
closed,  the  animal,  after  a  number  of  trials,  would  turn  to  the 
left.  One  specimen  which  had  come  to  turn  to  the  left  in 
the  first  fifty  trials,  turned  to  the  left  uniformly  thereafter 
in  the  following  fifty  trips.  The  left  side  was  then  blocked 
by  the  glass  and  in  the  next  fifty  trips  forty  were  successful. 
Finally  the  crayfish  made  but  one  error  in  fifty  trials. 
Further  changes  in  the  position  of  the  glass  plate  were  made, 
but  the  crayfish  after  a  number  of  trials  adjusted  itself  to 
the  new  condition. 

The  docility  of  the  crayfish  is  sho\\m  also  by  some  experi- 
ments of  the  writer  in  which  the  animals  were  trained  to  come 
for  food.  To  quote  from  a  previous  paper:  "At  first  I 
would  very  slowly  bring  a  piece  of  meat  held  in  a  fine  forceps 
near  the  antennules.  After  the  movements  of  the  antennules 
and  mouth  parts,  the  grasping  movements  of  the  chelipeds 
would  result  in  securing  the  meat.    After  some  trials  I  would 


ISG        PRIMITIVE  TYPES  OF  IXTF.LLIGENCE 

not  allow  Uie  meat  to  be  pulled  away  from  the  forceps  until 
the  craj'fish  struggled  awhile  to  secure  it ;  at  the  same  time  I 
moved  my  hand  about  so  as  to  accustom  the  animal  to  my 
movanoits.  There  is  a  strag^  between  the  instinct  to 
flee  &om  a  large  moving  object  and  the  instinct  to  secure  a 
SKVQiy  morsel  which  has  been  sdied.  With  careful  manage- 
ment the  latter  instinct  may  be  made  to  predominate  over 
the  form^  and  gradually  the  fear  of  one's  movements  be- 
comes much  reduced.  The  crayfish  finally  came  to  associate 
thea{^xrDach  of  my  hand  with  being  fed,  and  would  rear  up  and 
hold  out  its  larger  chdae  much  as  in  the  ordinary  posture  for 
defoise.  .  .  .  One  individual  would  greet  me  as  I  entered 
my  room  in  the  morning  by  raising  up  its  chelipeds  and 
coming  toward  me,  and  it  woidd  follow  me  about  as  I  went 
from  one  side  of  its  enclosure  to  the  other.  When  fed, 
bowevo*,  it  would  manifest  no  further  interest  in  my  move- 
moits." 

The  alHlity  of  hermit  ^rahs  to  form  associations  has  been 
prov^i  by  the  experimaits  of  Spaulding  on  Pagurus  longi^ 
carpus.  Several  specimens  <tf  this  active  species  were  placed  in 
«i  aquarium  supplied  with  running  water.  A  dai^  screen  was 
made  so  that  it  could  be  placed  in  the  middle  of  the  aquarium 
leaving  only  a  narrow  sKt  on  either  side  through  which  the 
oabs  could  pass  from  one  compartment  to  the  other.  As 
the  hermits  are  positivdy  |rfiototactic  they  tend  to  keep 
in  the  lifter  half  ol  the  aquarium,  and  to  make  the  lighting 
of  the  two  parts  as  diffaent  as  possible  one  side  of  the  aquar- 
ium was  covered  with  heavy  dark  paper.  When  the  screen 
was  put  in  the  aquarium,  and  all  the  crabs  placed  behind  it, 
they  quickly  made  for  the  openings  at  the  sides  of  the  parti- 
tion and  went  out  into  the  light.  Every  day  the  screen 
was  insoted  for  a  ffvea  interval  in  the  aquarium,  and  a 
piece  of  fish  put  behind  it.    The  number  of  crabs  entering 


PRIMITIVE  TYPES  OF  INTELLIGENCE         187 

the  darkened  part  after  the  food  was  noted  and  the  length  of 
time  required  for  the  crab  to  enter.  On  the  first  day  only 
three  out  of  the  thirty  crabs  used  availed  themselves  of  the 
food.  The  number  gradually  increased  on  succeeding  days, 
and  the  average  time  of  their  response  decreased  iitil;  on 
the  eighth  day,  all  the  crabs  but  one  entered  the  dark  en- 
closure, and  most  of  them  entered  with  little  delay. 

"After  a  few  days  of  this  treatment  immediately  upon 
the  insertion  of  the  screen  the  crabs  became  most  agitated, 
some  hurrying  and  scurrying  about,  others  making  almost 
directly  for  the  openings.'^  An  association  was  evidently 
formed  between  the  appearance  of  the  screen  and  the  ex- 
perience of  being  fed,  and  this  association  led  the  animals  to 
act  counter  to  their  natural  proclivity  to  seek  the  light. 

The  experiments  of  Drzewina  on  Pachygrapsus,  in  which 
it  was  shown  that  the  crabs  which  were  compelled  to  go 
through  a  certain  opening  in  order  to  get  nearer  the  light 
gradually  learned  the  way  and  arrived  more  quickly  in  the 
compartment  of  their  enclosure  which  was  most  illuminated, 
show  a  similar  power  of  forming  associations. 

In  the  Mollusca  we  meet  with  indications  of  intelligence  of 
a  primitive  sort  in  the  movements  of  gasteropods  such  as  the 
limpets,  which  have  the  faculty  of  making  considerable 
journeys  from  their  accustomed  stations  on  the  rocks  and 
returning  to  their  orginal  position.  Bethe  has  studied  the 
homing  of  limpets  and  has  come  to  the  conclusion  that  these 
animals  simply  follow  their  own  slimy  trails  and  are  guided 
back  to  their  resting  place  by  a  kind  of  chemotaxis.  No 
intelligence  is  required  by  the  limpets;  they  simply  obey 
a  blind  tropism. 

Lloyd  Morgan  experimented  with  limpets  by  removing 
them  for  some  distance  from  their  scars  on  the  rock  and 
noting  how  many  found  their  way  back  within  a  given  time. 


188         PRIMITIVE  TYPES  OF  INTELLIGENCE 

The  results  of  his  experiment  are  summed  up  in  the  following 
table: 


No.  removed 

Distance  in 

inches 

Number 

returned 

In  2  tides 

Je 

I  4  tides 

La1 

25 

6 

21 

.. 

21 

12 

13 

5 

.. 

21 

18 

10 

6 

2 

36 

24 

1 

1 

3 

While  the  majority  found  their  way  back  when  removed  but 
a  few  inches,  only  one  returned  from  a  distance  of  two  feet. 
The  return  of  the  limpets  was  watched  in  many  cases  and 
found  to  be  ''fairly,  but  not  quite  direct."  When  the 
limpets  reach  their  scar  "  they  twist  and  turn  about  so  as  to 
fit  down  in  the  normal  position,  which  is  constant.  When 
they  come  up  the  wrong  way  round  they  rotate  pretty 
rapidly  through  the  180  degrees  to  get  into  position." 

According  to  Bohn,  Patellas  when  they  have  remained  for 
some  time  in  a  certain  position,  whether  horizontal,  vertical 
or  oblique,  orient  themselves  in  a  similar  position  after  they 
have  been  removed  from  their  original  habitat.  Even  if 
they  are  allowed  to  remain  on  their  particular  scars  and  the 
rock  to  which  they  adhere  is  turned  in  another  position  they 
become  uneasy  and  crawl  around  until  they  find  a  niche  in 
which  they  may  lie  in  their  previous  orientation. 

It  is  among  the  active  and  highly  organized  cephalopods 
with  their  large  and  complex  nervous  centers  and  highly 
developed  organs  of  vision  that  we  naturally  expect  to  find 
the  highest  degree  of  psychic  development.  Unfortunately 
we  have  very  few  observations  on  this  head.  Schneider 
relates  that  a  young  octopus  which  be  observed  in  the 
Naples  aquarium  made  an  attack  on  a  hermit  crab  living 
within  a  shell  upon  which  were  several  anemones.  In 
approaching  the  crab  the  octopus  was  stung  by  the  net- 
tling cells  of  the  anemones  and  quickly  withdrew.    There- 


PRIMITIVE  TYPES  OF  INTELLIGENCE         189 

after  it  carefully  avoided  further  contact  with  the  crab. 
Older  octopi,  according  to  Schneider,  contrive  to  extract  the 
hermits  from  their  shells  without  being  stung.  The  ob- 
servations of  Schneider  on  the  young  of  the  octopus  were 
verified  by  von  Uexkull  in  Eledone  which  also  learned  to 
avoid  a  torpedo  from  which  it  had  received  an  electric 
shock. 

Kollmann  gives  an  acount  of  an  octopus  which  was  placed 
in  an  aquarium  with  a  large  lobster  and  several  other 
animals.  The  octopus  manoeuvred  constantly  in  order  to 
seize  the  lobster,  but  the  latter  was  on  the  alert  and  usually 
made  its  escape,  and  on  one  occasion  inflicted  a  severe  cut 
on  its  adversary's  arm.  The  lobster  was  finally  seized  in  an 
unwary  moment  and  surrounded  by  the  long  and  powerful 
arms  of  its  captor.  It  was  liberated  by  an  attendant  and 
placed  in  an  adjoining  aquarium  separated  from  the  first 
by  a  cement  partition  which  projected  about  2  cm.  above  the 
surface  of  the  water.  The  octopus  then,  although  the  lob- 
ster was  out  of  its  sight,  made  a  sudden  spring  over  the 
partition  and  soon  caught  and  overcame  its  prey. 

It  is  difficult  to  estimate  the  psychic  aspect  of  a  single 
act  such  as  this.  According  to  Kollmann  it  shows  that  the 
octopus  has  the  power  of  representing  the  absent  lobster  and 
of  remembering  where  it  was  placed;  but  it  is  not  safe  to  go 
farther  than  to  say  that  a  certain  amount  of  intelligence 
was  probably  involved. 

BIBLIOGRAPHY 

Bethe,  a.     Das  Nerv^ensystem  von  Carcinus  moenas  I,  Arch.  f. 

mik.  Anat.  50,  460  and  549,  '97;  II,  I.e.  51,  447,  '97. 
Drzewina,   a.   Les  reactions  adaptives  des  Crabes.     Bull.   Inst. 

G^n.  Psych.     8,  235,  '08. 
Kollmann,  J.     Aus  dem  Leben  der  Cephalopoden,  Vierteljahrschr. 

wiss.     Philos.  1,  '77. 


190        PRIMITIVE  TYPES  OF  INTELLIGENCE 

Morgan,  C.  L.     Animal  Behaviour,  London,  '00. 

Schneider,  G.  H.     Der  thierische  Wille,  '80. 

Spaulding,  E.  G.  An  Establishment  of  Association  in  Hermit  Crabs, 
Eupagurus  longicarpus.     Jour.  Comp.  Neur.  Psych.  14,  49,  '04. 

XJEXKtJLL,  J.  VON.  Physiologische  Untersuchungen  an  Eledone  mos- 
chata.     Zeit.  fur  Biol.     Vols.  28,  30,  31,  '92-'95. 

WiLLCOX,  M.  A.  Homing  of  Fissurella  and  Siphonaria.  Science, 
n.  s.  22,  90,  '05. 

Yerkes,  R.  M.  Habit  Formation  in  the  Green  Crab,  Carcinus  gran- 
ulatus.     Biol.  Bull.  3,  241,  '02. 

Yerkes,  R.  M.,  and  Huggins,  G.  E.  Habit  Formation  in  the  Craw- 
fish, Camharus  ajffinis.     Harvard  Psych.  Studies  1,  565,  '03. 

Yung,  E.  La  psychologie  de  I'escargot.  C.  r.  et  Trav.  Soc.  Helv. 
Sci.  Nat.  '93,  127. 


CHAPTER  X 
INTELLIGENCE  IN  INSECTS 

"Si  tons  les  actes  instinctifs  des  Insectes  portaient  constamment 
I'empreinte  4\adeiit  d'une  necessity  aveugle,  il  y  aurait  beaucoup 
moins  k  admirer  en  eux  qu'on  ne  le  fait  communemeiit.  Ce  qui  excite 
surtout  notre  surprise,  c'est  que  frequemment  ils  s'accommodent  aux 
circonstances,  et  que  leurs  actes  prennent  alors  une  telle  apparence 
de  raison,  qu'il  faut  y  regarder  de  pres  pour  ne  pas  les  attribuer  k 
une  veritable  combinasion  d'idees. — Lacordaire,  Introduction  d 
VEniomologie. 

In  the  insects  manifestations  of  intelligent  behavior  are 
much  more  common  and  more  striking  than  in  the  Crustacea 
and  molluscs.  It  is  a  general  rule  that  the  degree  of  intel- 
ligence in  these  forms  runs  parallel  with  the  degree  of  com- 
plexity and  perfection  of  their  instincts  and  with  the  degree 
of  development  of  the  nervous  system  and  sense  organs. 
Among  primitive  groups  of  insects  the  intelligence  manifested 
is  very  slight,  while  it  reaches  its  culmination  in  the  hymenop- 
tera  whose  instincts  have  long  been  objects  of  wonder  and 
admiration. 

The  power  of  associating  certain  appearances  with  food 
might  be  expected  to  occur  among  the  earliest  manifestations 
of  intelligence,  and  we  find  many  illustrations  of  this  ability 
even  among  the  more  primitive  insects.  Miss  Sondheim 
kept  a  damsel  fly  larva  in  a  dish  of  water,  where  it  was 
frequently  fed.  At  first  the  larva  scuttled  away  in  fear 
whenever  Miss  Sondheim  approached,  but  after  a  time  its 
timidity  was  overcome.  Later  it  became  so  tame  that  it 
would  take  flies  out  of  her  hand,  and  came  toward  her 
whenever  she  approached.    Finally  it  would  come  out  of  the 

191 


192  INTELLIGENCE  IN  INSECTS 

water  and  climb  upon  her  hand  in  order  to  get  the  food. 
Another  larva  which  was  worked  with  failed  to  profit  in 
the  least  from  repeated  efforts  to  train  it.  Forel  similarly 
trained  a  large  water  beetle,  which  at  first  fled  upon  his 
approach,  to  come  toward  him  for  food.  The  beetle  came 
to  eat  when  on  the  table,  whereas  naturally  it  feeds  only  in 
the  water,  but  it  still  retained  its  old  method  of  turning  over 
on  its  back  before  eating,  which  it  did  very  clumsily  when 
out  of  its  natural  element.  Lubbock  trained  a  wasp  to 
come  for  food,  and  finally  it  would  allow  itself  to  be  taken 
into  the  hand  and  stroked,  whereas  at  first  it  would  show 
strong  resentment  at  attempts  of  this  kind.  Very  similar 
results  were  obtained  by  Adlerz  in  a  species  of  sand  wasp. 

Mr.  J.  Wodsedalek  has  recently  made  an  extended  study 
of  the  formation  of  associations  in  the  May-fly  nymph 
Heptagenia  interpunctata  (Say) ,  which  is  very  common  near 
the  University  of  Wisconsin,  where  the  work  was  carried 
on.  Although  negatively  phototatic,  the  nymphs  were 
trained  to  go  toward  a  stone  (to  which  they  had  a  strong 
propensity  to  cling),  at  increasingly  great  distances  against 
the  rays  of  light,  until  finally  they  would  go  toward  it  at  a 
distance  much  greater  than  they  could  be  induced  to  do  at 
first.  They  were  also  trained  to  come  for  food,  and  by 
repeated  stirring  up,  several  lots  of  nymphs  came  to  be  so 
afraid  that  whenever  the  observer  approached  they  would 
scurry  about  with  every  appearance  of  great  alarm. 
Nymphs  placed  in  other  dishes  where  they  were  not  dis- 
turbed showed  practically  no  signs  of  fear.  It  is  of  inter- 
est to  find  in  these  primitive  insects  that  behavior  is 
modified  in  the  two  ways  which  in  higher  forms  we  should 
have  little  hesitation  in  regarding  as  indicative  of  pleasure 
and  pain. 

There  are  several  instances  of  the  "training"  of  ants. 


INTELLIGENCE  IN  INSECTS  193 

Formica  rufibarbis  is  a  very  pugnacious  species  and  the  odor 
of  one's  hand  readily  proyokes  it  to  a  fight.  Wasmann 
gradually  trained  a  worker  of  this  species,  offering  it  honey 
on  the  end  of  a  needle,  and  after  it  came  to  accept  the  food 
without  hesitation,  placing  the  honey  on  his  finger  where 
it  came  to  be  accepted  with  no  manifestation  of  fear  or 
hostility. 

Insects,  like  higher  animals,  learn  to  avoid  injurious  sub- 
stances which  they  at  first  attempted  to  use  for  food. 
Renter  placed  near  a  nest  of  ants  some  syrup  containing 
poison.  The  ants  partook  of  the  syrup  eagerly,  but  soon 
ejected  it  from  their  stomachs;  after  a  little  they  came  to 
avoid  the  syrup  although  numbers  of  them  w^ere  commonly 
near  it. 

The  ability  of  insects  to  find  their  way  back  to  their  nest 
or  home  is  developed  in  many  cases  to  a  very  remarkable 
degree.  Bethe,  possessed  of  the  idea  that  insects  are  reflex 
machines  incapable  of  learning  by  experience,  explains  this 
power  in  the  case  of  ants  as  an  instance  of  chemotaxis;  but 
in  the  bees  and  wasps  which  find  their  way  back  from  consid- 
erable distances  through  the  air,  where  scent  tracks  would 
not  persist,  he  is  driven  to  assume  some  mysterious  power, 
acting  in  a  manner  analagous  to  magnetic  force,  which 
guides  these  insects  to  their  goal.  Ants  have  the  instinct 
to  follow  the  scent  tracks  left  by  their  feet  in  going  from 
the  nest,  but  as  Cornetz  has  shown,  they  generally  do  not 
follow  these  at  all  closely,  and  usually  return  by  a  much 
more  direct  course  than  the  irregular  path  w^hich  is  commonly 
taken  in  their  outgoing  journeys  for  food.  The  power  of 
return  exhibited  by  bees  and  wasps  is  sho\\Ti  pretty  clearly 
by  the  experiments  of  Lubbock,  Buttel-Reepen,  the  Peck- 
hams,  Wagner  and  others  to  depend  upon  the  individual 
experience  of  these  insects.     The  homing  of  insects  takes 

13 


194 


INTELLIGENCE  IN  INSECTS 


place  in  essentially  the  same  way  as  the  homing  of  carrier 
pigeons,  and  involves  an  acquaintance  with  the  locality 
gained  by  previous  exploration.  The  Peckhams  found  that 
solitary  wasps  before  their  first  depai'ture  from  the  nest 
make  elaborate  "locality  studies/'  circling  around  the  nest 
in  wider  and  wider  courses  and  at  the  same  time  flying  higher 
and  higher  in  the  air.  Speaking  of  an  Ammophila  which 
for  some  time  previously  had  been  exploring  a  garden  in 


Fig.  13. — A  locality  study  of  the  wasp  Sphex.     (After  Peckham.) 

search  of  a  place  to  dig  a  nest,  the  Peckhams  say,  "  At  last 
a  spot  is  selected  and  she  begins  to  dig,  but  two  or  three 
times  before  the  work  is  completed  she  goes  away  for  a 
short  flight.  When  it  is  done,  and  covered  over,  she  flies 
away,  but  returns  again  and  again  within  the  next  few  hours 
to  look  at  the  spot  and,  perhaps,  to  make  some  little  altera- 
tion in  her  arrangements.  From  this  time  on,  until  the 
caterpillars  are  stored  and  the  egg  laid,  she  visits  her  nest 
several  times  a  day,  so  that  she  becomes  perfectly  familiar 
with  the  neighborhood,  and  it  is  not  surprising,  after  all, 
that  she  is  able  to  carry  her  prey  from  any  point  in  her 


INTELLIGENCE  IN  INSECTS  195 

territory  in  a  nearly  direct  line  to  her  hole — we  say  nearly 
direct,  for  there  was  almost  invariably  some  slight  mistake 
in  the  direction  which  made  a  Httle  looking  about  necessary 
before  the  exact  spot  was  found. 

"After  days  passed  in  flying  about  the  garden — going  up 
Bean  Street  and  down  Onion  Avenue,  time  and  time  again — 
one  would  think  that  any  formal  study  of  the  precise  locality 
of  a  nest  might  be  omitted,  but  it  was  not  so  with  our  wasps. 
They  made  repeated  and  detailed  studies  of  the  surroundings 
of  their  nests.  Moreover,  when  their  prey  was  laid  down 
for  a  moment  on  the  w^ay  home,  they  felt  the  necessity  of 
noting  the  place  carefully  before  leaving  it."  Similar 
"  locality  studies ''  varying  more  or  less  in  character,  are  made 
by  many  other  genera  of  wasps,  but  after  a  number  of 
flights  from  home  the  preliminary  circling  about  becomes 
gradually  reduced,  and  finally  the  insects  fly  away  in  a 
straight  line. 

The  role  of  visual  memory  is  shown  in  the  way  in  which 
wasps  are  disconcerted  by  a  change  in  the  region  about  their 
nests  made  during  their  absence.  ''Aporus  faciatus,'^  say 
the  Peckhams,  "entirely  lost  her  way  when  we  broke  off 
the  leaf  that  covered  her  nest,  but  found  it,  without  trouble, 
when  the  missing  object  was  replaced.  All  the  species  of 
Cerceris  were  extremely  annoyed  if  we  placed  any  new  object 
near  their  nesting  places.  Our  Ammophila  refused  to  make 
use  of  her  burrow  after  we  had  drawn  some  deep  lines  in  the 
dust  before  it.  The  same  annoyance  is  exhibited  when 
there  is  any  change  made  near  the  spot  upon  which  the  prey 
of  the  wasp,  whatever  it  may  be,  is  deposited  temporarily." 
Even  a  slight  change  so  disconcerted  the  wasps  that  they  were 
obliged  to  hunt  for  a  long  time  before  recovering  their  prey. 

Belt  has  recorded  a  very  interesting  case  of  a  wasp,  Polistes 
camifex,  which  had  caught  a  caterpillar  too  large  to  be  carried 


196  INTELLIGENCE  IN  INSECTS 

to  its  nest.  After  chewing  the  caterpillar  for  some  time 
the  wasp  bit  it  in  two  and  rolled  up  one  of  the  parts  in  order  to 
carry  it  away.  As  if  it  had  in  mind  retm*ning  for  the  other 
piece  the  wasp  circled  about,  forming  larger  and  larger 
circles,  then  departed  for  a  distance,  but  returned  as  if  to 
get  another  look  at  the  situation  and  flew  away.  In  less 
than  two  minutes  the  wasp  again  appeared  on  the  scene, 
having  probably  disposed  of  its  burden  in  its  nest.  It  had 
difficulty  in  finding  the  remainder  of  the  caterpillar  and  re- 
turned repeatedly  to  the  same  seed  pods  near  which  the  prey 
was  located.  Whenever  in  flying  about  it  came  near  the 
pods  it  would  alight  and  continue  the  search  on  foot.  Its 
persevering  efforts  were  rewarded  by  the  discovery  of  the 
remainder  of  the  caterpillar;  then  it  seized  its  prey  eagerly, 
and  ''  as  if  there  was  nothing  more  to  come  back  for,  flew 
straight  to  its  nest  without  taking  any  further  note  of  the 
locality.'* 

Similar  behavior  is  shown  in  the  trial  flight  of  bees.  When 
young  bees,  or  bees  which  have  been  carried  into  a  new  place, 
make  their  first  excursions  from  the  hive  they  circle  about  as 
they  rise  through  the  air  before  venturing  very  far  away.  Only 
after  having  flown  back  and  forth  from  the  hive  several 
times  do  they  finally  come  to  dispense  with  their  preliminary 
movements  of  exploration.  If  young  bees  are  removed 
from  the  hive,  even  for  only  a  short  distance,  before  they 
have  made  their  trial  flight,  they  fail  to  find  their  way  back. 
On  the  other  hand,  old  bees  may  be  removed  for  a  long 
distance  and  almost  all  succeed  in  returning.  The  distance 
from  which  bees  are  able  to  find  their  way  home  depends  upon 
the  character  of  their  surroundings,  and  particularly  upon 
the  distances  they  have  been  in  the  habit  of  going  for 
honey.  Romanes  took  bees  from  a  hive  which  was  situated 
near  the  sea,  carried  them  in  a  boat  a  short  distance  from 


INTELLIGENCE  IN  INSECTS  197 

the  shore  and  set  them  free.  That  they  might  be  easily 
recognized  the  bees  were  previously  made  to  walk  in  bird 
lime  and  the  hive  was  carefully  watched  for  their  return, 
but  none  found  their  way  home.  Another  lot  of  bees  was 
then  liberated  on  the  shore  not  far  from  the  hive.  In  this 
region  there  were  no  flowers,  consequently  it  was  one  not 
frequented  by  the  bees,  and  they  were  no  more  successful 
in  finding  their  way  back  than  in  the  first  experiment.  A 
number  of  bees  were  next  carried  inland  where  they  had  been 
in  the  habit  of  foraging  for  honey,  and  liberated;  nearly  all 
quickly  found  their  way  back  to  the  hive. 

Buttel-Reepen  found  that  if  a  hive  is  carried  into  a  new 
locality  a  considerable  distance  from  its  original  situation 
and  concealed  among  shrubbery  or  between  buildings  so 
that  it  cannot  be  seen  from  a  distance,  and  the  old  bees  are 
removed  before  they  have  made  a  trial  flight,  they  usually 
fail  to  return  even  if  they  have  been  taken  but  one-hundred 
feet  from  the  hive.  If  the  hive  is  removed  to  a  short  distance 
of  a  few  rods,  numbers  of  the  bees  return  to  the  original 
position  of  the  hive  and  fly  about  as  if  in  search  of  the  hive, 
although  the  latter  may  be  in  plain  sight.  The  memory 
of  bees  for  the  position  of  an  object  is  apparently  better  than 
their  memory  of  the  object  itself.  In  this  the  insect  mind 
acts  rather  differently  from  that  of  higher  animals  for  which 
the  object,  wherever  situated,  is  the  thing  that  usually 
determines  action. 

Insects  seem  chained  down  t^  topographical  relations  and 
free  themselves  from  their  guiding  influences  only  with 
difficulty.  This  is  illustrated  by  Fabre's  experiments  on  the 
mason  bee,  Chalcidoma  muraria.  During  the  absence  of  the 
bee  Fabre  removed  her  nest  the  distance  of  one  met». 
The  bee  returned  to  the  old  locahty  of  the  nest,  but  failed 
to  discover  her  own.    TMien  another  nest  was  placed  in  the 


198  INTELLIGENCE  IN  INSECTS 

position  of  the  old  one  the  bee  would  work  upon  it  as  if 
unaware  of  the  substitution.  The  same  trait  is  shown  in 
the  interesting  experiments  of  Turner  on  the  homing  of  bur- 
rowing bees.  Melissodes,  the  first  form  worked  with,  digs 
holes  in  the  ground  and  makes  excursions  from  the  nest  at 
quite  regular  intervals.  During  the  bee's  absence  a  rect- 
angular piece  of  white  paper  with  a  hole  in  the  center  was 
placed  over  the  nest  so  that  the  hole  of  the  paper  coincided 
with  that  of  the  burrow.  The  bee  when  returning  circled 
about  the  nest,  hovered  over  the  paper,  and  then  circled 
about  again;  after  repeating  such  performances  for  two  min- 
utes she  entered  the  nest.  On  her  next  return  she  hovered 
about  for  half  a  minute  and  made  her  entrance.  During 
her  next  absence  a  hole  was  made  in  the  ground  about  four 
inches  away  and  covered  by  the  white  paper  as  before, 
while  a  piece  of  watermelon  rind  with  a  hole  in  the  center 
was  placed  over  the  burrow.  When  the  bee  returned  she 
hovered  over  the  melon  rind  and  circled  about  for  a  minute 
as  if  appreciating  that  things  were  not  quite  as  they  should 
be  and  then  entered  her  nest.  The  melon  rind  was  then 
removed  and  a  rectangular  piece  of  white  paper  was  arched 
over  the  nest  so  as  to  form  a  covering  open  at  either  end. 
The  piece  of  paper  with  the  hole  in  the  center  was  left  where 
it  was  in  the  preceding  experiment.  When  the  bee  returned 
she  circled  around  for  about  a  minute  and  then  went  into 
the  hole  in  the  middle  of  this  paper.  The  insect  was  deceived, 
but  only  temporarily,  for  she  quickly  came  out  of  the  artificial 
hole,  entered  one  end  of  the  tent-like  covering,  and  found 
her  hole.  The  arrangements  were  left  undisturbed  during 
the  next  three  flights  of  the  bee,  and  the  insect  foimd  her 
nest  with  little  loss  of  time,  as  she  did  also  when  the  tent 
was  turned  at  right  angles  to  its  previous  position. 
In  another  experiment  the  tent  was  removed  for  a  short 


INTELLIGENCE  IN  INSECTS  199 

distance  and  a  black  piece  of  paper  with  a  hole  in  the  center 
was  placed  over  the  nest  so  that  the  hole  in  the  paper  lay 
directly  over  the  opening  of  the  nest.  The  bee  returned  and 
entered  her  nest  after  hovering  for  but  a  few  seconds  over  the 
black  paper.  Finally  all  the  accessories  were  swept  away 
and  the  region  around  the  nest  covered  uniformly  with 
green  grass  leaving  the  opening  uncovered.  On  her  return 
the  bee  was  disconcerted,  circled  about  the  nest  for  about 
two  minutes  and  finally  entered  it. 

The  hole  of  another  species  of  burrowing  bees  was  found 
near  one  of  a  series  of  bricks  which  formed  the  border  of  a 
walk.  Near  the  hole  was  the  cover  of  a  bottle.  During  the 
absence  of  the  bee  Turner  punched  holes  of  the  same  diameter 
as  the  bee's  nest  and  bearing  the  same  relation  to  the  other 
bricks  as  the  nest  did  to  the  brick  near  it.  The  top  of  the 
bottle  was  placed  near  one  of  these  artificial  holes.  On  her 
return  the  bee  alighted  some  distance  away  and  came  along 
the  series  of  bricks  until  she  encountered  the  hole  near  the 
bottle  cover  when  she  immediately  plunged  into  it.  She 
quickly  recognized  her  error,  withdrew,  and  soon  found  her 
own  hole.  During  her  second  absence  holes  were  punched 
in  front  of  several  more  bricks  on  either  side  of  the  nest,  but 
the  bee  on  her  return  once  more  entered  the  hole  near  the 
bottle  cover.  She  emerged,  hovered  over  the  spot,  and 
again  entered  the  same  hole,  but  soon  came  out  and  found 
her  own  nest.  Evidently  the  bottle  cap  served  as  a  land- 
mark indicating  the  position  of  her  nest.  The  environment 
of  the  different  holes  was  so  similar  that  a  change  in  the 
position  of  this  one  object  changed  the  principal  feature  of 
the  local  topography. 

There  is  no  support  here  for  Bethe's  theory  of  a  mysteri- 
ous force,  the  assumption  of  a  dead  reckoning  process,  or 
the  '^kinaesthetic  reflex"  of  Pieron.     The  results  are  only 


200  INTELLIGENCE  IN  INSECTS 

explicable  on  the  assumption  that  the  bee  has  a  memory  of 
space  relations  and  guides  her  flight  accordingly.  This  con- 
clusion is  supported  by  the  results  of  Turner's  experiments 
on  the  homing  of  the  mud  dauber,  which  are  in  principle 
the  same  as  the  foregoing,  although  differing  much  in  detail. 
In  bees  and  wasps  the  memory  of  locality  is  shown  by  their 
returning  repeatedly  to  the  same  spot  for  food.  Forel  in 
experimenting  on  the  power  of  vision  and  the  formation  of 
associations  in  bees  made  use  of  variously  colored  paper 
flowers  on  each  of  which  he  placed  a  drop  of  honey.  The 
artificial  flowers  were  placed  among  some  Dahlias  which  the 
bees  were  visiting.  A  red  paper  flower  was  brought  near  a 
bee  resting  on  a  Dahlia,  but  the  bee  was  at  first  so  occupied 
in  gathering  honey  that  she  could  be  induced  to  visit  the 
red  flower  only  when  the  honey  was  brought  within  reach 
of  her  proboscis.  The  bee's  back  was  then  marked 
with  red  paint  in  order  that  she  could  be  distinguished 
from  other  bees.  When  the  bee  returned  from  the  hive 
she  went  straight  for  the  artificial  red  flower,  then  to  a  blue 
artificial  flower  with  a  yellow  center  and  finally  back  to  the 
first.  Another  bee  which  visited  a  white  artificial  flower 
was  painted  yellow.  On  her  return  from  the  hive  she  flew 
to  the  same  artificial  flower  and  then  visited  two  others  and 
did  not  return  to  the  Dahlias.  Later  the  bulk  of  the  bees 
w^hich  for  a  long  time  had,  with  few  exceptions,  ignored  the 
artificial  flowers  seemed  to  have  their  attention  directed  to 
them  by  other  visitors  and  threw  themselves  upon  the  arte- 
facts in  swarms.  After  they  had  devoured  the  honey  the 
bees  began  to  go  back  to  the  Dahlias,  but  when  colored 
artificial  flowers  devoid  of  honey  and  hence  lacking  an 
attractive  odor  were  placed  among  the  plants,  many  bees 
began  to  visit  them  and  examine  them  carefully  as  if  they 
expected  to  find  honey  there.    These  facts,  as  Forel  rightly 


INTELLIGENCE  IN  INSECTS  201 

concludes,  ''can  only  be  explained  by  an  association  of  space 
form  and  color  memories  with  memories  of  taste." 

We  are  certainly  justified  in  concluding  that  insects  are 
not  mere  reflex  machines  incapable  of  learning  by  experience. 
They  can  form  associations  very  quickly  in  many  cases. 
They  give  evidence  of  memory.  They  have  a  remarkable 
ability  for  retaining  impressions  of  topographical  relations. 
We  may  not  be  compelled  to  admit  that  they  have  ideas, 
but  it  must  be  granted,  I  think,  that  a  wasp  which  after 
cutting  a  caterpillar  in  two  and  carrying  away  one  part, 
came  back  and  searched  diligently  for  the  remainder,  re- 
tained somehow  an  impression  of  the  missing  part  and  its 
location.  If  out  of  sight  it  was  not  out  of  mind.  The  hunt- 
ing of  the  wasp  is  instinctive  and  when  we  see  a  wasp 
flitting  about  here  and  there  in  a  feverish  search  for  prey 
we  cannot  assume  that  it  carries  in  its  mind  an  image  of  the 
object  of  its  pursuit.  But  the  case  is  different  with  a  wasp 
which  has  secured  its  prey  and  comes  back  to  find  it.  The 
prey  and  its  position  are  represented  by  some  sort  of  "en- 
gram"  in  the  nervous  center  of  the  wasp,  which  is  formed 
by  the  various  stimuli,  optical,  olfactory  and  tactual,  which 
resulted  from  the  encounter.  If  the  wasp  does  not  have  an 
idea  of  its  prey  it  has  something  which  plays  a  role  similar 
to  that  of  ideas  in  ourselves.  As  the  wasp  when  it  has 
disposed  of  the  second  moiety  of  the  caterpillar  no  longer 
returns,  its  mental  content  is  evidently  changed  by  having 
carried  the  part  to  its  nest.  If  there  is  something  represent- 
ing "part-of-caterpillar-among-leaves"  that  leads  the  wasp 
on  its  hunt,  we  may  conclude  that  there  is  also  something 
corresponding  to  "  part-of-caterpillar-now-in-nest "  which 
prevents  further  search. 

I  realize  that  one  is  on  treacherous  ground  in  trying  to 
interpret  the  workings  of  the  insect  mind.    Forel,  whose 


202  INTELLIGENCE  IN  INSECTS 

judgments  on  animal  psychology  are  usually  conservative, 
attributes  to  insects  an  ''ability  to  instinctively  draw  in- 
ferences from  analogy/'  Some  of  the  facts  adduced  are  the 
following:  After  bees  had  been  trained  to  come  to  artificial 
flowers  of  a  certain  color  for  honey  they  deserted  the  Dahlias 
upon  which  they  had  been  working  and  began  to  work 
upon  other  artefacts  of  different  colors  and  in  various 
positions.  The  bee  may  be  supposed,  according  to  Forel, 
if  we  interpret  him  correctly,  to  go  through  with  a  mental 
process  corresponding  to  "This  appearance  means  honey; 
therefore  this  other  similar  appearance  likewise  means  honey; 
I  will  investigate  it."  The  behavior  of  the  bee  may  indicate 
a  step  toward  rational  procedm*e,  but  we  are  hardly 
justified  in  assuming  that  any  act  of  comparison  between 
similar  flowers  takes  place  in  the  insect's  mind.  A  certain 
appearance  has  been  associated  with  the  act  of  sucking 
honey.  This  association  leads  the  bee  to  visit  the  same 
artificial  flower  again;  or  we  may  say  that  this  object  tends 
to  set  in  action  the  honey-getting  activities.  If  the  same 
object  causes  the  return  of  the  bee  we  do  not  appeal  to  any 
inference  from  analogy.  If  now  a  similar  object  provokes 
the  visit  of  the  bee,  it  may  mean  simply  that  the  stimulus 
is  sufficiently  like  the  first  to  set  the  honey-getting  activities 
in  motion.  The  bee  gets  a  different  perception  from  the 
second  object,  but  it  does  not  necessarily  recognize  that  it  is 
different  from  and  at  the  same  time  similar  to  the  first. 
What  appears  in  many  cases  to  be  reasoning  from  analogy, 
involving  judgments  of  likeness,  is  really  based  on  nothing 
more  than  lack  of  discrimination.  While  granting  that  a 
simple  act  of  inference  may  be  performed  by  the  bee,  the 
facts  do  not,  I  think,  require  us  to  conclude  that  it  actually 
is  performed. 

Another  case  involving  a  decided  approach  to  reason, 


INTELLIGENCE  IN  INSECTS 


203 


according  to  another  prominent  student  of  conservative 
judgment,  Professor  Lloyd  Morgan,  is  furnished  by  the 
observations  of  the  Peckhams  on  a  soUtary  wasp  Ammophila 
which,  after  filUng  up  its  hole  with  dirt  even  with  the  surface 
of  the  ground,  picked  up  a  small  pebble  in  her  mandibles  and 


Fig.  14. 


-The  wasp  Ammophila  using  a  stone  to  pound  down  the  dirt 
in  its  hole.     (After  Peckham.) 


used  it  to  pound  in  the  loose  dirt  placed  in  the  hole,  ''  Before 
we  could  recover  from  our  astonishment  at  this  performance," 
write  the  Peckhams,  "she  had  dropped  her  stone  and  was 
bringing  more  earth,  and  in  a  moment  we  saw  her  pick 
up  the  pebble  and  again  pound  the  earth  into  place  with  it. 
Once  more  the  whole  process  was  repeated,  and  then  the 
little  creature  flew  away."     According  to  Morgan,   "here 


204  INTELLIGENCE  IN  INSECTS 

we  have  intelligent  behavior  rising  to  a  level  to  which  some 
would  apply  the  term  rational.  For  the  act  may  be  held  to 
afford  evidence  of  the  perception  of  the  relation  of  the  means 
employed  to  an  end  to  be  attained,  and  some  general  con- 
ception of  purpose."  Truly  a  ''tool  using  animal!"  But 
in  estimating  the  psychic  aspect  of  the  performance  we 
must  bear  in  mind  that  the  act  is  one  which  borders  closely 
upon  the  normal  instinctive  behavior  of  the  insect.  The 
seizure  of  pebbles  in  the  mandibles  and  the  packing  in  of 
dirt  are  parts  of  the  instinctive  process  of  filling  up  the  hole. 
The  wasp  combines  two  features  of  its  hole-filling  instinct  in 
a  rather  unusual  w^ay.  Does  she  really  perceive  the  relation 
of  means  to  end?     I  am  not  so  sure  that  she  does. 

Many  readers  who  peruse  this  chapter  will  miss  the 
wonderful  accounts  of  insect  ingenuity  which  they  may 
have  expected  to  find.  The  literature  of  insect  behavior 
contains  these  in  abundance.  Run  through  the  files  of 
Nature,  Science  Gossip,  Der  Zoologische  Garten,  La  Nature, 
The  Zoologist,  the  older  numbers  of  the  Annals  and  Magazine 
of  Natural  History,  and  The  American  Naturalist,  the 
various  entomological  journals,  and  works  of  travellers 
w^ith  a  leaning  toward  natural  history;  and  peruse  the  many 
volumes  that  have  been  written  on  the  instincts  and  intelli- 
gence and  reasoning  power  of  animals,  and  you  will  encounter 
an  enormous  mass  of  material,  some  of  it  carefully  recorded, 
much  of  it  not,  more  of  it  vitiated  by  anthropomorphic 
interpretation  which  one  cannot  help  feeling  has  biased  the 
observer  in  his  account  of  the  facts,  but  the  data  that  can  be 
employed  in  drawing  conclusions  regarding  the  degree  of 
intelligence  shown  by  the  forms  observed  is  disappointingly 
small.  Animals  are  observed  overcoming  obstacles  or  meet- 
ing unusual  situations,  and  the  occurrence  is  straightway 
recorded  as  an  illustration  of  sagacity,  something  for  which 


INTELLIGENCE  IN  INSECTS  205 

*'mere  instinct"  will  not  account.  In  most  cases  there 
has  been  no  previous  study  of  the  animars  behavior  with  a 
view  to  ascertaining  the  nature  and  limitations  of  its  instinc- 
tive performances.  Ants  are  observed  to  build  bridges  over 
water  or  other  substances  which  they  are  desirous  of  crossing, 
by  bringing  grains  of  sand  or  bits  of  earth  and  dropping  them 
until  they  can  effect  a  passage.  The  ants  are  then  credited 
with  ingenuity,  reason,  imagination  and  other  mental 
qualities  which  human  beings  would  employ  to  overcome  a 
similar  difficulty.  And  taken  by  themselves  the  facts 
seem  to  justify  such  conclusions.  Light  on  the  matter, 
however,  is  thrown,  as  in  so  many  other  cases,  by  a  study  of 
the  creature's  instincts,  which  show^s  that  the  apparent  feat 
of  engineering  is  the  result  of  an  instinctive  propensity  of  the 
ant,  slightly  modified  perhaps  to  meet  the  particular  occasion. 
Wasmann,  in  an  instructive  experiment,  placed  on  the 
nest  of  Formica  sanguinea  a  watch  glass  filled  wdth  water 
in  the  center  of  which  was  a  sort  of  island  on  w^hich  were 
isolated  a  few  pupae.  The  ants  brought  sand  and  threw 
it  into  the  watch  glass  until  they  had  formed  a  passage  way 
to  the  pupse,  which  were  then  carried  away.  Ingenuity, 
surely,  one  is  tempted  to  say!  But  the  next  experiment 
inspires  caution.  A  watch  glass  with  no  island  and  no  pupse 
was  placed  in  the  nest.  This  was  filled  like  the  previous  one. 
At  least  one  important  factor  in  the  ants'  activities  is  the 
instinct  to  cover  offending  objects  which  cannot  be  removed 
— an  instinct  analagous  to  that  of  bees,  which  leads  them  to 
cover  over  with  propolis  objects  too  large  to  remove  from  the 
hive.  If  a  dog  performed  similar  actions  in  endeavoring  to 
reach  an  object  otherwise  inaccessible  we  should  be  justified 
in  attributing  to  the  animal  a  considerable  degree  of  intelli- 
gence. Some  intelligence  may  have  been  involved  in  the 
behavior  of  the  ants,  but  not  necessarily  more  than  a  very 


206  INTELLIGENCE  IN  INSECTS 

little.  The  celebrated  instance  adduced  by  Leuckart  of 
ants  bringing  grains  of  sand  to  cover  a  streak  of  tobacco 
solution  across  their  trail  may  be  explained  in  a  similar  way. 

Another  instance.  Kirby  and  Spence  record  a  case 
communicated  by  a  German  artist,  whom  they  were  assured 
was  "a  man  of  strict  veracity.'^  A  dung  beetle,  having 
made  a  pellet  for  the  reception  of  its  eggs,  found  that  it  was 
unable  to  roll  the  pellet  out  of  a  depression  into  which  it 
had  fallen.  The  beetle  then  repaired  to  a  dung  heap  near  by 
and  returned  with  three  companions,  with  whose  assistance 
the  ball  was  rolled  out,  after  which  the  three  beetles  took 
their  departure.  This  is  one  of  the  evidences  from  which 
insects  are  considered  to  be  "able  to  communicate  and 
receive  information,  which,  in  whatever  way  effected, 
would  be  impracticable  if  they  were  devoid  of  reason.'' 

Blanchard  in  his  ''Metamorphoses,  Moeurs  et  Instincts 
des  Insectes,"  gives  an  account  of  a  very  similar  performance, 
which  the  author  considers  to  evince  ''  une  intelligence  de  la 
situation  vraiment  ^tonnante,  et  une  facility  de  communica- 
tion entre  les  individus  de  la  m^me  esp^ces,  plus  surprenante 
encore.''  Here  again  we  must  take  into  consideration  the 
normal  instinctive  behavior  of  these  insects.  Frequently 
two  or  more  beetles  are  found  rolling  the  same  ball.  As 
Fabre  has  shown  in  his  careful  studies  of  the  sacred  scarab, 
an  allied  beetle  with  similar  habits,  such  associations  are 
dependent  on  quite  different  motives  than  the  altruistic 
desire  of  rendering  assistance.  The  helpful  comrades  turn 
out  to  be  bent  on  getting  the  ball  for  themselves.  Sometimes 
they  abandon  the  task  voluntarily;  often  they  wage  a  com- 
bat with  the  original  owner.  The  succoring  of  a  comrade  in 
distress  is  only  an  appearance  which  a  fuller  study  of  the 
habits  of  these  insects  places  in  a  quite  different  light. 

These  cases   illustrate   a   common   source   of  erroneous 


INTELLIGENCE  IN  INSECTS  207 

conclusions  in  the  study  of  animal  intelligence,  one  which 
is  responsible  for  a  great  multitude  of  stories  of  doubtful 
value.  Observations  may  have  been  recorded  faithfully 
and  accurately,  but  where  they  have  not  been  made  by  in- 
vestigators thoroughly  acquainted  with  the  general  behavior 
of  the  forms  observed,  mistaken  interpretations  are  almost 
certain  to  arise.  On  the  other  hand,  one  is  tortured  by  the 
feeling  that  our  experimental  methods  often  fail  to  give  us  a 
true  measure  of  an  animaPs  possible  attainments,  and  that 
it  is  just  in  meeting  exceptional  situations  which  occur  in 
the  animal's  natural  course  of  life  that  the  highest  manifesta- 
tion of  its  intelligence  is  reached. 

A  factor  which  markedly  affects  the  behavior  of  many 
insects,  especially  the  social  ones,  is  the  influence  of  numbers. 
Small  stocks  of  bees,  according  to  Buttel-Reepen,  lose  their 
spirit  and  allow  themselves  to  become  the  prey  of  moths 
and  robber  bees  which  are  not  so  easily  tolerated  by  larger 
stocks.  They  work  with  less  vigor  and  fight  with  less 
courage,  as  if  conscious  of  the  fact  that  in  numbers  there  is 
strength  and  that  their  number  is  small.  Forel  says  of  ants 
that  "the  courage  of  each  ant  grows  in  proportion  to  the 
number  of  her  comrades  or  friends  and  diminishes  in  just 
the  proportion  that  she  is  isolated.  .  .  .  The  same 
worker  ant,  which  in  the  midst  of  her  associates,  is  ready 
to  face  death  ten  times  over,  when  alone  and  twenty  steps 
away  from  her  nest,  becomes  cowardly,  avoids  the  least 
danger,  and  seeks  safety  in  flight  from  an  ant  much  weaker 
than  herself."  In  regard  to  Formica  sanguinea  Wasmann 
states  that  "if  a  numerous  population  inhabits  a  rotten 
fir  stump,  on  the  surface  of  which  we  find  some  of  the 
ants  running  about,  a  gentle  kick  will  at  once  call  forth  a 
whole  army  ready  for  the  fray.  In  a  moment  the  whole 
surface  of  the  stump  is  covered  with  thousands  of  ants 


208  INTELLIGENCE  IN  INSECTS 

furiously  hurrying  to  and  fro.  But  if  the  colony  is  weak, 
the  same  kick,  which  at  other  times  calls  forth  an  army, 
will  have  the  contrary  effect.  The  ants  which  just  before 
were  running  about  the  surface  disappear  through  the^  en- 
trances of  the  nest  as  if  by  magic,  and  deathlike  quiet 
succeeds.^' 

Wagner  in  his  valuable  studies  on  the  behavior  of  bumble 
bees,  has  observed  that  the  bees  of  well  populated  nests 
are  much  more  pugnacious  than  those  from  nests  containing 
few  members.  In  regard  to  the  household  activities  of 
bumble  bees  Wagner  remarks  that  "Wenn  eine  Hummel 
allein  mit  der  Ausbesserung  des  Nestes  beschaftigt  ist,  so 
erscheint  ihre  Tatigkeit,  obwohl  wie  stets  geschaftig,  doch 
wenig  energisch  und  intensiv.  Kaum  hat  sich  jedoch  der 
ersten  Hummel  eine  zweite  angeschlossen,  so  nimmt  bei 
gleichen  sonstigen  Bedingungen  die  Energie  der  Arbeit  zu 
und  wachst  bei  jedem  neu  hinzutretenden  Mitarbeiter  mit 
voller  Augenscheinlichkeit.  Gans  dieselbe  Erscheinung  habe 
ich  auch  bei  Grabwespen  beobachtet.^'  The  "brooding^' 
of  the  cells,  gathering  food,  and  various  other  activities 
increase  in  vigor  with  increase  in  numbers,  so  that  the  popula- 
tion of  the  large  nests  seems  larger  and  that  of  the  small 
nests  smaller  than  it  is  in  reality. 

The  influence  of  numbers  upon  the  industry  and  courage 
of  social  insects  has  been  adduced  as  a  proof  of  a  high  degree 
of  intelligence.  That  a  populous  ant  colony  will  make  an 
attack  upon  a  neighboring  nest,  while  a  colony  with  few 
members  shows  a  disinclination  for  warfare,  may  give  the 
appearance  of  a  conscious  realization  of  strength  and  a 
power  of  reflecting  upon  the  probable  issue  of  an  encounter. 
As  Wagner  has  pointed  out,  the  explanation  of  such  pheno- 
mena may  lie  in  the  effect  upon  the  individual  of  various 
stimuli  caused  by  its  associates.    The  ant  or  bee  commonly 


INTELLIGENCE  IN  INSECTS  209 

receives  from  other  members  of  its  community  numerous 
olfactory,  tactile,  and  perhaps  auditory  stimulations  to 
which  it  instinctively  responds.  These  stimulations  pro- 
duce a  condition  of  general  nervous  irritability  which 
spurs  the  insects  on  to  activity  and  makes  them  ready  to 
combine  in  an  attack  upon  an  enemy.  Whether  or  not 
similar  effects,  as  Wagner  contends,  are  produced  in  solitary 
insects — and  I  am  not  convinced  that  they  are,  from  the 
evidence  adduced — it  is  undoubtedly  true  that  social 
insects  are  dependent  upon  the  stimuli  received  from  the 
cooperation  of  others  to  a  remarkable  degree.  The  confusion 
produced  by  the  loss  of  a  queen  and  the  gradual  languishing 
of  a  swarm  in  which  no  queen  can  be  supplied,  show  how 
sensitive*  are  bees  to  changes  in  their  social  environment. 
Among  bees,  ants  and  termites  signs  of  anger  by  one  in- 
dividual may  awaken  the  whole  community  to  a  high  pitch 
of  excitement.  Each  individual  then  serves  to  arouse  the 
others,  and  the  larger  the  community  the  greater  the  mass 
effect. 

Closely  associated  in  many  cases  with  the  influence  of 
numbers  is  the  effect  of  imitation.  The  activities  of  insects 
not  only  arouse  the  energies  of  their  fellows,  but  they  also 
direct  their  efforts  and  in  this  manner  secure  cooperation 
toward  a  common  end.  Ants  keep  together  in  their  forag- 
ing expeditions  and  often  follow  the  " scouts''  which  act 
as  leaders,  guiding  the  expedition  to  the  nest  to  be  pillaged. 
"In  artificial  nests,"  says  Wheeler,  "one  usually  sees  a 
particular  activity  started  by  one  or  a  few  workers,  which 
have  more  initiative  or  respond  more  quickly  to  a  change 
of  conditions  than  the  great  bulk  of  the  colony.  The  move- 
ments of  such  individuals  attract  the  attention  of  others  in 
their  immediate  neighborhood  and  these  forthwith  proceed  to 
imitate  their  more  alert  companions.    Then  the  activity 

14 


210  INTELLIGENCE  IN  INSECTS 

spreads  like  a  conflagration  till  it  has  seized  on  most  or  all 
the  members  of  a  community."  It  is  in  their  attacks  upon 
enemies  that  imitation  in  ants  is  especially  marked.  An 
attack  by  an  ant  is  the  signal  for  others  to  join  in  the  fray. 
Sometimes  a  strange  ant  may  be  tolerated  in  a  nest  until 
it  happens  to  arouse  the  animosity  of  one  of  the  members, 
when  various  others  fall  to  and  help  to  dispatch  the  intruder. 

Wasmann  states  that  several  beetles  of  the  species  Dinarda 
dentata  were  received  as  guests  in  a  nest  of  Formica  sanguinea 
and  had  lived  there  for  some  time  and  propagated.  He 
then  placed  in  the  nest  a  specimen  of  an  allied  species  of 
Dinarda  which  was  attacked  and  killed.  This  aroused  the 
killing  propensities  of  the  other  ants  which  fell  upon  the 
Dinarda  dentatas,  and  the  guests  which  had  been  kept  for 
so  long  met  their  fate.  That  ants  in  their  treatment  of 
aphids  are  influenced  by  imitation  is  indicated  by  the  fact, 
signalized  by  Forel  and  Adlerz,  that  Formica  sanguinea, 
which  very  rarely  make  use  of  honey  dew  as  food,  readily 
adopts  the  custom  of  its  slaves  upon  perceiving  them  solicit 
the  aphids  for  their  sweet  exudation.  Slave  making  ants 
readily  adopt  guests  which  are  received  in  a  friendly  manner 
by  their  slaves,  although  they  would  otherwise  be  apt  to 
attack  them,  and  slaves  in  turn  are  disposed  to  be  friendly 
to  the  guests  which  they  perceive  to  be  tolerated  by  their 
masters. 

In  treating  of  insect  intelligence  we  shall  find  it  instructive 
to  consider  its  failures  as  well  as  its  exceptional  manifesta- 
tions. As  Forel  has  remarked  insects  are  exceedingly  stupid 
as  regards  everything  not  closely  related  to  their  instinctive 
interests.  But  even  when  the  latter  are  involved,  they 
usually  fail  to  make  the  simplest  and  most  obvious  inferences. 
A  striking  case  is  furnished  by  the  Amazon  slave-making 
ant,  Polyergus  rufescens,  which  on  account  of  the  remarkable 


INTELLIGENCE  IN  INSECTS  211 

military  tactics  displayed  in  its  frequent  warlike  expeditions 
against  other  ants  might  be  expected  to  rank  among  the 
most  intelligent  of  insects.  Polyergus  is  dependent  upon 
its  slaves  for  food,  having  almost  completely  lost  the  in- 
stincts for  food  taking  from  its  long  habituation  to  parasitic 
habits.  There  is  no  physical  peculiarity  which  prevents 
these  ants  from  getting  their  own  food,  and  occasionally  they 
take  liquid  food  when  by  chance  the  mouth  parts  are  brought 
in  contact  with  it.  As  Lubbock,  Wasmann  and  others 
have  shown  the  Amazons  if  deprived  of  their  slaves  will 
starve  to  death  in  the  midst  of  plenty  without  making 
the  least  effort  to  secure  food  of  their  own  accord.  As 
Wasmann  observes,  ''their  hunger  does  not  compel  them 
like  other  animals  to  seek  for  food  themselves,  but  only  to 
beg  food  of  other  ants  by  taps  of  their  feelers.  The  sensitive 
perception  of  food  placed  immediately  before  them,  in  spite 
of  their  feeling  of  hunger,  does  no  longer  excite  in  them  the 
natural  impulse  of  tasting  it.''  We  should  naturally  expect 
that  a  creature  possessing  the  rudiments  of  intelligence  would 
be  able  to  associate  the  appearance  and  odor  of  food  with 
the  act  of  feeding.  Possibly  the  Amazons  might,  if  skillfully 
managed,  be  taught  to  form  this  association,  but  that  they 
do  not  do  so  under  the  stress  of  starvation  shows  how  poor 
in  resources  is  the  emmet  mind. 

To  the  same  purport  we  may  cite  the  following  quotation 
concerning  ants  from  Sir  John  Lubbock:  "In  order  to  test 
their  intelligence,  it  has  always  seemed  to  me  that  there  was 
no  better  way  than  to  ascertain  some  object  that  they  would 
clearly  desire,  and  then  to  interpose  some  obstacle  which  a 
little  ingenuity  would  enable  them  to  overcome.  Following 
up,  then,  the  preceding  observations,  I  placed  some  larvae 
in  a  cup  which  I  put  on  a  slip  of  glass  surrounded  by  water, 
but  accessible  to  the  ants  by  one  pathway  in  which  was  a 


212  INTELLIGENCE  IN  INSECTS 

bridge  consisting  of  a  strip  of  paper  2/3  inch  long  and  1/3 
inch  wide.  Having  then  put  a  Lasius  niger  from  one  of 
my  nests  to  these  larvae,  she  began  carrying  them  off,  and 
by  degrees  a  number  of  friends  came  to  help  her.  I  then, 
when  about  twenty-five  ants  were  so  engaged,  moved  the 
little  paper  bridge  slightly,  so  as  to  leave  a  chasm,  just  so 
wide  that  the  ants  could  not  reach  across.  They  came  and 
tried  hard  to  do  so;  but  it  did  not  occur  to  them  to  push  the 
paper  bridge,  though  the  distance  was  only  about  1/3  inch, 
and  they  might  easily  have  done  so.  After  tr3dng  for  about 
a  quarter  of  an  hour,  they  gave  up  the  attempt  and  returned 
home.    This  I  repeated  several  times. 

"Then,  thinking  that  paper  was  a  substance  to  which 
they  were  not  accustomed,  I  tried  the  same  with  a  bit  of 
straw  1  inch  long  and  1/8  inch  wide.  The  result  was  the 
same.     I  repeated  this  more  than  once. 

"Again  I  suspended  some  honey  over  a  nest  of  Lasius 
flavus  at  a  height  of  about  1/2  inch,  and  accessible  only  by 
a  paper  bridge  more  than  10  feet  long.  Under  the  glass  I 
then  placed  a  small  heap  of  earth.  The  ants  soon  swarmed 
over  the  earth  on  to  the  glass,  and  began  feeding  on  the  honey. 
I  then  removed  a  little  of  the  earth,  so  that  there  was  an 
interval  of  about  1/3  of  an  inch  between  the  glass  and  the 
earth;  but,  though  the  distance  was  so  small,  they  would  not 
jump  down,  but  preferred  to  go  round  by  the  long  bridge. 
They  tried  in  vain  to  stretch  up  from  the  earth  to  the  glass, 
which,  however,  was  just  out  of  their  reach,  though  they 
could  touch  it  with  their  antennae;  but  it  did  not  occur  to 
them  to  heap  the  earth  up  a  little,  though  if  they  had  moved 
only  half  a  dozen  particles  of  earth  they  would  have  secured 
for  themselves  direct  access  to  the  food.  This,  however, 
never  occurred  to  them.  At  length  they  gave  up  all  attempts 
to  reach  up  to  the  glass,  and  went  round  by  the  paper  bridge. 


INTELLIGENCE  IN  INSECTS  213 

I  left  the  arrangement  for  several  weeks,  but  they  continued 
to  go  round  by  the  long  paper  bridge." 

The  facts  above  stated  should  render  us  suspicious  of 
conclusions  regarding  the  high  degree  of  intelligence  which 
ants  have  been  supposed  to  manifest  in  certain  of  their 
activities,  especially  in  their  powers  of  communication,  in 
their  military  manceuvers,  and  in  the  keeping  of  slaves  and 
guests.  A  foraging  ant  finds  some  sugar,  or  a  dead  insect  too 
heavy  to  carry  to  the  nest,  and  she  goes  home,  communicates 
by  means  of  striking  with  the  antennae  with  other  ants,  and 
then  returns  with  several  companions  to  her  prize.  Or  it  may 
be  that  some  of  the  "scouts"  of  a  marauding  species  discover 
a  nest  of  a  species  preyed  upon,  and  after  visiting  her 
own  nest  and  making  her  report,  guides  an  expedition  of 
warriors  to  the  habitation  of  the  enemy.  The  older  writers 
on  ants  and  some  of  the  modem  ones  have  made  much  of 
their  power  of  communication,  and  in  reading  their  accounts 
one  might  almost  be  led  to  believe  that  ants  have  a  language 
with  a  large  vocabulary,  and  hold  elaborate  dissertations 
on  the  food  discovered,  the  whereabouts  of  their  enemies, 
their  strength,  and  the  most  feasible  way  in  which  to  conduct 
an  attack.  That  some  power  of  communication  exists  has 
been  abundantly  shown,  but  for  the  most  part  it  consists 
of  signs  instinctively  made  under  certain  conditions  and 
which  are  instinctively  responded  to  by  other  ants.  In 
spite  of  the  valuable  investigations  of  several  of  the  foremost 
m3Tmecologists  our  knowledge  of  ant  "language"  is  very 
imperfect.  Among  the  actions  which  have  been  considered 
to  be  involved  in  communication  are  striking  with  the  anten- 
nae, butting  with  the  head,  opening  the  jaws,  beating  the 
floor  with  the  abdomen,  and  the  production  of  sounds  by 
various  kinds  of  apparatus  for  stridulation.  What  the 
particular  things  may  be  which  are  signified  by  these  various 


214  INTELLIGENCE  IN  INSECTS 

acts — ^if  they  do  signify  particular  things — ^has  not  been 
discovered.  The  general  upshot  of  a  long  series  of  experi- 
ments by  Lubbock  is  that  an  ant,  while  having  the  power 
of  leading  others  to  food,  is  unable  to  inform  its  comrades 
as  to  the  whereabouts  of  food  so  that  they  may  reach  it  by 
themselves.  Lubbock  allowed  ants  to  take  food  and  go  back 
to  the  nest;  when  these  ants  returned  accompanied  by  several 
companions  the  former  were  caught  in  order  to  discover  if 
their  companions  would  then  be  able  to  find  the  food  alone. 
This  they  rarely  succeeded  in  doing,  although  they  would 
scurry  about  in  various  directions  as  if  seeking  something. 
These  facts  seem  to  show  that  the  communication  of  the 
location  of  food  or  other  desirable  objects  is,  as  Romanes 
has  expressed  it,  "in  the  nature  of  some  sign  amounting  to 
no  more  than  a  'follow  me.^  ^^  While  ants  may  not  be  able 
to  talk  about  things  in  their  sign  language,  they  apparently 
express  their  different  feelings  and  inclinations  in  ways  which 
are  intelligible  to  other  ants.  Wasmann  has  compiled  a 
sort  of  vocabulary  of  signs  made  by  the  antennae — a  "Worter- 
buch  der  Fiihlersprache,'^  which  is  about  as  extensive  as 
Mr.  Garner's  language  of  apes.  According  to  the  vigor 
and  frequency  of  the  strokes  of  the  antennae,  and  the  part 
of  the  body  stroked,  the  ant  which  is  addressed  may  be  im- 
portuned for  food,  warned  of  danger,  or  induced  to  cooperate 
with  the  communicant  in  various  activities.  Naturally  one 
is  inclined  to  be  skeptical  regarding  what  seems  like  many 
of  the  romantic  tales  of  animal  psychology,  but  Wasmann 
is  expressing  little  more  than  the  general  opinion  among 
students  of  ant  life  regarding  the  powers  of  communication 
possessed  by  these  insects,  and  the  conclusions  of  so  careful 
and  experienced  a  myrmecologist  and  one  withal  so  little  to 
be  suspected  of  a  tendency  to  "humanizing  the  brute'' 
are  deserving  of  the  most  careful  consideration. 


INTELLIGENCE  IN  INSECTS  215 

The  power  of  communication  among  bees  is  very  limited. 
As  the  result  of  a  long  series  of  experiments  on  honey  bees 
Lubbock  has  concluded  that  these  insects  do  not  lead  one 
another  to  places  where  they  find  food.  After  numerous 
observations  and  experiments  on  bumble  bees  Wagner  has 
come  to  a  similar  conclusion  for  these  forms.  In  both 
hive  bees  and  bumble  bees  the  angry  hum  of  a  few  bees  is 
taken  up  by  others  and  a  sort  of  communication  of  anger 
spreads  through  the  group.  Similarly  a  note  of  distress,  the 
''heulen,"  of  the  hive  bee  which  frequently  follows  upon 
the  loss  of  the  queen,  spreads  from  the  bees  which  first 
discover  the  loss.  Another  note  is  predominant  in  swarming 
time,  which  sometimes  evokes  swarming  activities  in  neigh- 
boring hives  (Buttel-Reepen).  In  bumble  bees  where  the 
language  of  sound  is  apparently  more  simple,  the  hum  of  the 
wings,  according  to  Wagner,  serves  solely  as  a  warning  of  the 
presence  of  danger — "  die  Huromeln  mit  Hilfe  ihrer  Flugel 
nur  von  drohender  Gefahr  und  von  nichts  anderem  Kunde 
geben  konnen."  All  of  the  varieties  of  sound  which  the 
bumble  bees  make  with  the  aid  of  their  wings  have  not  the 
least  effect  upon  their  comrades,  with  the  single  exception  of 
the  peculiar  note  emitted  in  time  of  danger  which  serves 
most  efficiently  to  arouse  other  inhabitants  of  the  nest. 
There  is  nothing  in  the  communication  of  ants  or  bees  that 
calls  for  the  exercise  of  much  intelligence.  Their  language, 
like  the  language  of  animals  everywhere,  consists  solely  of 
instinctively    made    and    instinctively    recognized    signs. 

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216  INTELLIGENCE  IN  INSECTS 

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Heft.  26,  '99. 

Comparative  Studies  in  the  Pyschology  of  Ants  and  of  Higher 

Animals  (Trans,  from  2nd.  ed.),  St.  Louis,  '05. 
Wheeler,  W.  M.    Ants,  N.  Y.,  '10. 


CHAPTER  XI 
INTELLIGENCE  IN  THE  LOWER  VERTEBRATES 

''To  the  minnow  every  cranny  and  pebble,  every  quality  and 
accident  of  its  little  native  creek  may  have  become  familiar;  but 
does  the  minnow  understand  the  ocean  tides?" — Thomas  Carlyle. 

If  an  extra  mundane  observer  were  ignorant  of  the  evolu- 
tion of  the  vertebrates  beyond  the  Silurian  or  the  Devonian 
epochs  it  is  doubtful  if  he  would  pick  out  these  animals  as 
the  ones  destined  to  surpass  all  others  in  psychic  develop- 
ment. The  numerous  species  of  highly  organized  cepha- 
lopods  that  thronged  the  seas,  the  trilobites  with  their 
highly  developed  organs  of  vision,  the  gigantic  eurypterids 
that  crawled  over  the  bottom  of  the  shallow  oceans,  the 
crustaceans,  the  terrestrial  arachnids  and  the  rapidly 
evolving  group  of  insects  might  all  have  been  regarded  as 
having  as  much  promise  of  future  psychic  development  as  the 
back-boned  "winners  of  life's  race."  And  most  of  the 
branches  of  the  vertebrate  tree  really  developed  no  further 
than  their  invertebrate  competitors.  From  among  the 
diverging  branches  of  this  phylum  one  only  contained  the 
stock  that  led  to  the  mammals  and  culminated  in  man. 

A  comparative  anatomist  looking  back  upon  the  course  of 
evolution  might  have  said:  The  vertebrates  were  obviously 
the  forms  with  the  most  promising  psychological  future. 
Many  of  these  ancient  forms  doubtless  possessed  a  cerebral 
cortex,  a  sort  of  appendix  to  the  central  nervous  system, 
whose  especial  business  it  is  to  take  care  of  the  establishment 
of  associations.  The  opportunity  was  open  to  them  through 
the  increase  in  size  and  complexity  of  the  association  centers 

218 


INTELLIGENCE  IN  LOWER  VERTEBRATES    219 

for  a  career  of  an  almost  imlimited  mental  development. 
The  molluscs  and  the  arthropods,  on  the  other  hand,  have  a 
different  sort  of  a  brain.  They  have  no  cerebral  cortex ;  there- 
fore they  cannot  form  associations  and  consequently  they  are 
but  more  or  less  complicated  "reflex  machines."  So  with 
certain  of  the  vertebrates,  the  bony  fish  whose  cerebral 
cortex  is  represented  by  a  membrane  of  non-nervous  tissue 
over  the  basal  ganglionic  centers,  they  too  must  be  chained 
down  to  the  routine  life  of  reflexes  and  instincts,  with  no 
power  of  learning,  no  ability  to  profit  by  experience.  So 
our  comparative  anatomist  might  have  argued  and  so  indeed 
some  comparative  anatomists  have  argued.  This  contention 
as  we  have  seen  is  far  from  justified  in  the  arthropods,  and 
we  shall  see  that  it  is  equally  groundless  as  regards  the 
vertebrates  with  no  cerebral  cortex. 

Every  angler  can  doubtless  furnish  evidence  of  the  learning 
of  fishes.  In  trout  streams  that  have  been  much  frequented 
the  fish  become  much  more  wary  of  the  bait  than  at  first, 
and  some  of  the  old,  experienced  fishes  can  be  induced  only 
with  great  difficulty  to  take  the  line.  On  the  other  hand, 
certain  fish  will  allow  themselves  to  be  caught  and  hauled 
out  of  the  water  repeatedly  without  conquering  their  pro- 
pensity to  dart  at  the  bait. 

No  one  can  read  very  much  in  comparative  psychology 
without  frequently  encountering  the  story  of  Mobius'  pike 
— a  story  which  the  professor  was  fond  of  repeating  in 
his  lectures  and  which  came  to  be  looked  forward  to  as  a 
regular  annual  event  by  his  students.  This  celebrated  pike 
was  kept  in  a  part  of  an  aquarium  separated  by  a  glass  plate 
from  an  adjoining  part  which  contained  several  minnows. 
The  pike  made  frequent  dashes  for  the  minnows  and  each 
time  received  a  bump  against  the  glass  plate.  After  about 
three  months  of  attempts  to  catch  the  minnows  the  pike 


220    INTELLIGENCE  IN  LOWER  VERTEBRATES 

became  convinced  that  his  efforts  were  fruitless  and  they 
were  given  up.  The  glass  partition  was  then  taken  away. 
The  pike  which  had  come  to  associate  darting  at  the  minnows 
with  bumps  on  its  nose  left  the  minnows  unmolested  there- 
after, being  apparently  unaware  of  the  removal  of  the  im- 
pediment to  catching  its  prey.  The  interpretation  of  the 
experiment  of  Mobius  was  questioned  by  Bateson,  whose 
experiments  on  the  intelligence  of  fishes  gave  in  general 
negative  results,  but  it  was  confirmed  by  the  investigations  of 
Triplett  on  the  educability  of  the  perch.  Triplett  used  an 
aquarium  divided  in  the  middle  by  a  glass  plate;  in  one  com- 
partment he  placed  a  male  and  a  female  perch  for  a  period 
of  thirty  minutes  three  times  a  week,  after  which  they  were 
removed  and  fed  on  w^orms.  Soon  after  the  two  perch  were 
put  in  one  compartment  some  minnows  were  introduced 
into  the  other.  The  perch  immediately  began  the  pursuit 
and  frequently  butted  their  heads  against  the  glass,  especially 
when  the  minnows  drew  near.  The  efforts  of  the  fish, 
especially  the  female,  were  very  vigorous,  but  near  the  end 
of  the  period  both  had  given  up  the  chase.  In  the  next 
trial  the  perch  vigorously  pursued  the  minnows,  but  with 
somewhat  less  energy  than  in  the  first  experiment.  For  a 
month  the  behavior  of  the  perch  continued  much  the  same. 
The  minnows  and  perch  were  then  kept  in  the  two  parts 
of  the  aquarium  as  in  the  experiments  of  Mobius  and  remained 
there  a  week.  The  perch  seldom  collided  with  the  partition, 
although  they  watched  the  minnows  frequently.  When 
angle  worms  were  placed  on  the  other  side  of  the  partition 
the  perch  dashed  violently  against  the  glass  for  some  time 
in  their  efforts  to  reach  them.  They  had  learned  to  avoid  the 
minnows,  but  the  suggestion  aroused  by  the  worms  on  which 
thev  were  regularly  fed  proved  too  strong  for  them.  After 
the  perch  had  ceased  their  futile  efforts  to  catch  the  minnows 


INTELLIGENCE  IN  LOWER  VERTEBRATES    221 

one  of  the  latter  was  placed  in  the  compartment  of  the  larger 
fish.  It  was  treated  at  first  much  as  before,  but  after  a  little 
"  the  perch  became  more  demonstrative  toward  it,  but  re- 
strained themselves  from  striking,  so  long  as  it  quietly- 
avoided  them/'  Allien  the  minnow  made  a  quick  movement 
the  perch  were  more  apt  to  dart  after  it  and  on  one  occa- 
sion it  had  to  be  rescued.  After  a  time  the  pursuit  of  the 
minnows  became  less  eager  and  although  they  were  followed 
the  instinct  to  dart  at  them  was  generally  inhibited. 

The  formation  of  an  association  in  relation  to  the  glass 
partition  was  shown  by  the  fact  that  when  the  partition  was 
removed  the  fish  behaved  much  as  if  it  still  remained.  "The 
male  swam  out  to  the  place,  stopped,  made  little  bumps 
forward  as  if  expectng  to  strike  the  usual  obstruction,  and 
was  plainly  at  a  loss.  He  then  turned  and  swam  down  as 
if  following  the  glass.''  Often  the  fish  would  swim  to  the 
mark  where  the  plate  had  been  and  then  turn  back. 

These  experiments  showed  that  the  natural  instinct  of  the 
perch  to  charge  after  the  minnow  was  inhibited,  although  not 
completely  so,  and  that  the  fish  came  to  associate  the  appear- 
ance of  a  certain  region  of  the  aquarium  with  the  experience 
of  being  bumped.  Pieron  found  that  a  cyprinoid  fish, 
Carassius  auratus,  would  snap  at  live  worms  placed  in  a  glass, 
tube,  but  the  number  of  attacks  in  a  given  time  gradually 
grew  less  on  the  following  days.  The  glass  tube  was  placed 
in  the  water  for  twenty-seven  minutes  each  time.  In  the 
first  experiment  the  fish  made  117  attacks,  on  the  three 
successive  days  the  attacks  were  58,  38,  and  25  in  number. 
Several  days  later  the  number  of  attacks  had  diminished 
still  more.  The  tube  was  then  left  in  the  aquarium  and  the 
fish  soon  came  to  ignore  it  entirely,  and  even  refused  to  eat 
the  worms  when  they  were  placed  free  in  the  water.  When 
the  number  of  attacks  on  the  tube  diminished  they  could  be 


222   INTELLIGENCE  IN  LOWER  VERTEBRATES 

augmented  if  the  tube  was  placed  in  a  different  part  of  the 
aquarium  or  by  feeding  the  fish  with  a  few  of  the  worms. 
That  fish  readily  associate  certain  stimuli  with  food  has 
been  shown  by  several  observers.  Herrick  found  that 
catfish  which  at  first  would  seize  pieces  of  cotton  which  were 
brought  in  contact  with  its  barbels  would  gradually  cease 
to  react  to  them.  After  a  few  trials  the  fish  would  make  a 
movement  toward  the  cotton,  but  it  was  soon  checked. 
Finally  the  cotton  could  be  rolled  over  the  barbels  or  other 
parts  of  the  body  without  eliciting  any  response.  The 
fish  probably  recognized  the  cotton  thi-ough  the  sense  of 
touch  as  it  made  little  difference  if,  at  any  time,  red  cotton 
was  substituted  for  white.  Washburn  and  Bentley  in  their 
experiments  on  the  creek  chub  showed  that  this  fish  is  able 
to  associate  different  colors  such  as  red  and  green  with  food. 
The  chub  in  one  set  of  experiments  was  fed  out  of  a  pair  of 
red  forceps  which  were  let  into  the  water  alongside  of  a  green 
pair.  Although  a  meal  worm  was  placed  in  the  red  pair  the 
fish  often  snapped  at  the  green  forceps  first,  but  after  a  num- 
ber of  trials  the  green  forceps  were  ignored.  When  the  fish 
had  been  given  a  number  of  worms  the  forceps  were  put  in 
empty.  The  fish  continued  to  snap  at  the  red  pair  and 
avoid  the  green  regardless  of  their  relative  positions.  Red 
and  green  forceps  of  different  degrees  of  brightness  were 
used  and  precautions  were  taken  to  eliminate  the  in- 
fluence of  odor,  but  in  the  last  40  experiments  in  which  no 
food  was  placed  in  either  forceps  and  the  relative  position 
of  the  forceps  exchanged  after  each  trial  the  fish  snapped  at 
the  red  pair  every  time.  It  is  probable,  therefore,  that  the 
chub  is  able  to  distinguish  colors  as  opposed  to  mere  differ- 
ences in  brightness — although  the  proof  of  this  is  not  en- 
tirely conclusive — and  it  is  very  evident  that  it  is  able  to 
form  associations  with  a  fair  degree  of  rapidity. 


INTELLIGENCE  IN  LOWER  VERTEBRATES    223 

Many  fish  can  easily  be  trained  to  come  toward  one  for 
food.  I  have  trained  goldfish  so  that  they  would  swim 
toward  my  hand  to  be  fed,  when  I  put  it  over  the  aquarium, 
and  I  find  upon  enquiry  that  many  others  have  done  the 
same.  A  sunfish  which  I  kept  for  several  months  would 
take  bits  of  snail  meat  out  of  my  fingers;  whenever  my 
hand  was  brought  near  the  water  the  sunfish  would  approach 
and  hold  its  mouth  near  the  surface  in  readiness  to  snap 
at  the  food.  I  gradually  accustomed  the  fish  to  jump  out 
of  the  water  for  the  food  to  a  distance  of  nearly  three  inches. 
I  would  often  put  my  hand  near  the  water  as  if  I  were 
holding  a  piece  of  meat.  The  sunfish  would  almost  invari- 
ably come  to  the  surface  in  a  position  of  readiness  for  a 
leap,  but  it  would  not  jump  at  my  empty  fingers.  A  piece 
of  dark  colored  snail  meat  not  much  larger  than  a  pin's 
head  or  a  small  black  mark  on  the  end  of  my  finger  would  be 
sufficient,  however,  to  cause  a  jump.  I  noticed  also  that 
the  fish  came  to  be  very  attentive  to  my  general  movements, 
and  whenever  I  drew  near  the  aquarium  it  would  be  found 
pointed  toward  me  with  its  head  near  the  glass;  and  if  I 
walked  around  the  aquarium  the  fish  would  faithfully  follow. 

Reighard  in  an  investigation  of  warning  coloration  in  fishes 
fed  a  colony  of  snappers  with  small  fishes  of  the  genus 
Atherina.  Some  of  the  Atherinas  were  stained  red  and 
others  were  unstained,  some  of  the  red  ones  were  rendered 
unpalatable  by  putting  in  their  mouths  parts  of  the  ten- 
tacles of  a  large  jelly  fish,  which  were  plentifully  supplied 
with  nettling  cells.  Red  Atherinas  were  readily  taken 
when  thrown  into  the  water,  but  those  which  had  been  made 
unpalatable  with  the  tentacles  of  the  jelly  fish,  while  snapped 
up  at  first,  were  generally  avoided  after  a  few  trials.  There- 
after the  snappers  avoided  the  red  Atherinas  which  had  not 
been  rendered  unpalatable.    They  had  come  to  associate  the 


224    INTELLIGENCE  IN  LOWER  VERTEBRATES 

red  color  with  the  irritation  caused  by  the  tentacles  so  that 
the  red  came  to  be  a  "warning  color."  Subsequently  un- 
colored  Atherinas  were  given  them  which  were  for  the  most 
part  instantly  taken.  Red  Atherinas  were  offered  after 
an  interval  of  eight  days  when  they  were  very  sparingly 
taken  and  then  refused  entirely,  and  again  after  an  interval 
of  twenty  days  when  they  remained  entirely  untouched. 

There  are  a  number  of  instances  of  fishes  coming  to  be  fed 
when  a  bell  was  rung  or  some  other  sound  made,  but  it  is 
probable  as  Kreidl  has  shown  in  one  instance  that  the  fish 
associated  the  food  with  the  appearance  of  the  person 
making  the  sound  rather  than  with  the  sound  itself. 

The  memory  of  topographical  relations  seems  to  be  well 
developed  in  certain  fishes  as  in  certain  insects.  It  is  mani- 
fested most  clearly  in  those  forms  which  have  a  more  or 
less  fijced  habitation  or  which  build  a  nest  for  their  eggs. 
A  good  illustration  of  this  faculty  is  afforded  by  a  species 
of  Goby  studied  by  Mile.  Goldsmith.  This  species,  Gohius 
minutus,  is  commonly  found  in  tide  pools  under  a  shell  of 
some  bivalve  mollusc  where  it  may  lie  half  buried  in  the 
sand.  That  the  fish  recognizes  its  shell  by  sight  is  shown 
by  the  fact  that  when  Mile.  Goldsmith  drove  a  specimen 
from  under  its  shell  and  placed  the  aquarium  where  it  was 
kept  in  the  dark  the  fish  did  not  succeed  in  finding  its  shell 
during  twenty-four  hours:  when  light  was  admitted  it  dis- 
covered the  shell  at  once.  The  experiment  was  repeated 
many  times  with  different  individuals,  with  the  same  result. 

The  ability  of  Gobius  to  learn  a  certain  path  to  its  shell 
is  shown  in  the  following  experiment :  A  goby  was  placed  in 
an  aquarium  divided  in  the  middle  by  a  glass  plate  which 
left  a  narrow  passage  way  at  one  end.  The  shell  of  the 
goby  was  placed  in  one  compartment  C  and  the  fish  was 
driven  through  the  passage  way  into  the  adjoining  part  of 


INTELLIGENCE  IN  LOWER  VERTEBRATES    225 

the  aquarium.  In  its  endeavors  to  reach  its  nest  the  goby 
swam  against  the  glass  partition  ten  times  and  then  found 
the  passage.  It  was  then  driven  from  its  shell  into  the 
adjoining  compartment  again.  It  now  rammed  its  head 
against  the  partition  six  times,  then  found  the  passage  and 
entered  its  shell.  In  the  next  trial  the  fish  made  only  three 
or  four  attempts  to  go  through  the  glass  plate,  and  after 
this  it  found  the  passage  way  at  once.  In  a  short  time  the 
fish  had  learned  the  way  to  its  nest  and  thereafter  followed 
it  with  little  hesitation. 

After  this  set  of  trials  the  goby,  now  left  to  itself,  began 
to  explore  the  aquarium  of  its  own  initiative,  by  making 
excursions  farther  and  farther  from  the  nest,  keeping  close 
to  the  outer  wall  and  returning  each  time  by  the  same  route 
by  which  it  set  out.  When  it  arrived  at  the  partition  it 
bumped  against  it,  but  after  two  or  three  trials  it  stopped 
a  little  short  of  the  barrier  and  continued  to  do  so  in  its 
subsequent  excursions. 

After  the  habit  of  avoiding  the  glass  plate  had  become 
well  established  the  plate  was  removed.  The  goby  never- 
theless continued  to  follow  the  old  path  in  its  journeys  to 
and  from  the  nest.  Even  when  the  fish  had  happened  two 
or  three  times  to  cross  the  place  where  the  partition  had 
been,  it  still  persisted  in  taking  the  old  round-about  course 
from  one  part  of  the  aquarium  to  the  other.  Finally  after 
the  fish  had  crossed  directly  several  times  the  old  habit 
became  gradually  broken  up  and  the  fish  paid  no  attention 
to  the  place  where  the  partition  had  been  located.  In  one 
interesting  experiment  Mile.  Goldsmith  filled  the  shell  of  a 
goby  with  mastic  and  placed  it  in  its  old  position.  The 
goby  came  to  its  shell  and  endeavored  to  get  under  it,  trying 
first  its  accustomed  point  of  entrance,  and  then  making 
attempts  to  dig  under  it  in  various  other  places.     It  dug 

15 


226   INTELLIGENCE  IN  LOWER  VERTEBRATE^ 

each  time  in  a  new  place  as  if  bearing  in  mind  its  failures 
at  other  points.  After  a  time  the  goby  seemed  to  become 
discouraged  and  left  the  shell,  but  returned  at  times  and 
resumed  its  attempts  to  enter.  After  an  hour  and  a  half 
the  goby  gave  up  its  efforts  entirely.  The  next  day  the 
mastic  was  removed  and  the  shell  placed  in  its  original 
position.  After  several  hours  the  goby  had  not  entered 
the  shell  and  swam  by  it  for  a  long  time  without  giving  it 
the  least  attention.  The  shell  was  then  removed  to  another 
part  of  the  aquarium.  As  soon  as  the  goby  perceived  the 
shell  it  quickly  made  for  it  and  installed  itself  under  it  as 
if  it  had  discovered  a  new  shell  instead  of  the  old  one.  With 
the  goby  as  with  the  mason  bee,  Chalcidoma,  the  place 
which  a  thing  occupies  is  its  chief  recognition  mark.  The 
same  shell  in  a  new  place  was  for  the  goby  a  new  object, 
with  promise  of  being  a  suitable  domicile  which,  it  had  come 
to  recognize,  the  shell  in  the  old  place  was  not. 

In  this  connection  the  experiment  of  Lloyd  Morgan  on 
the  behavior  of  a  male  stikleback  is  of  interest.  "A  nest 
had  been  built  on  a  round  glass  bell  jar  which  stood  near 
a  window.  Some  aquatic  vegetation  grew  in  the  tank, 
and  the  nest  was  built  on  the  window  side.  An  experiment 
was  made  by  turning  the  large  bell  jar  through  a  right  angle. 
The  male  stickleback  searched  for  its  nest  in  the  old  direc- 
tion on  the  window  side,  that  is  to  say,  the  same  position  in 
reference  to  the  incidence  of  the  light.  The  search  was,  of 
course,  fruitless,  and  a  new  nest  was  begun  in  this  position. 
Presently  the  old  nest  was  discovered,  and  was  then  vig- 
orously destroyed  in  just  the  same  way  as  the  nest  of  a  rival 
is  pulled  to  pieces  and  scattered.  Here  a  new  incidence  of 
light  and  new  direction  of  shadows  seemed  to  have  com- 
pletely transformed  the  visual  situation." 

The  Amphibia,  notwithstanding  the  fact  that  they  have 


INTELLIGENCE  IN  LOWER  VERTEBRATES    227 

a  cerebral  cortex,  which  is  lacking  in  the  bony  fishes,  cannot 
be  said  to  surpass  the  latter  in  the  development  of  intelli- 
gence. The  tailed  amphibians  are  notoriously  sluggish  and 
give  every  appe^ance  of  extreme  stupidity.  Nevertheless 
they  show  a  remarkably  acute  and  delicate  sensibility  and  a 
power  of  very  rapid  action  on  occasions  which  is  beHed  by 
their  general  appearance  and  customary  slow  movements. 
Our  knowledge  of  their  intelligence  is  slight.  Few  have 
attempted  to  educate  such  apparently  unpromising  subjects 
and  neither  Brehm,  Buchner  nor  Romanes  gives  any  re- 
markable cases  illustrative  of  their  sagacity.  Professor 
Whitman,  in  his  interesting  account  of  the  behavior  of 
Necturus,  considers  that  this  animal  has  a  certain  amount  of 
intelligence  which  is  involved  in  its  learning  by  experience 
how  to  direct  its  movements,  but  he  assumes  nothing  be- 
yond this.  Pieron  has  observed  that  if  worms  enclosed  in 
a  glass  tube  are  introduced  into  an  aquarium  wdth  specimens 
of  Triton,  the  amphibians  make  a  large  number  of  attempts 
to  seize  the  worms  before  showing  any  noticeable  falling 
off  in  the  number  of  their  fruitless  efforts;  fish  under  similar 
circumstances  learn  to  avoid  the  worms  much  more  quickly. 
In  another  experiment  a  quadrangular  flask  containing 
several  worms  was  placed  on  its  side  for  a  half  hour  a  day 
in  an  aquarium,  containing  six  tritons.  Sometimes  the 
animals  found  the  entrance  by  chance  and  ate  some  of  the 
worms.  With  the  exception  of  one  individual  which  seemed 
to  learn  the  way  fairly  quickly  there  was  no  marked  increase 
in  the  facility  with  which  the  tritons  entered  the  flask.  The 
experiment,  while  not  thorough-going,  indicates  that  asso- 
ciations in  these  animals  are  usually  formed  but  slowly. 

Among  the  Anura  there  have  been  several  studies  on  the 
intelligence  of  frogs  and  toads.  These  animals  are  capable 
of  forming  simple  associations,  but  their  powers  are  very 


228   INTELLIGENCE  IN  LOWER  VERTEBRATES 

limited.  Abbot  placed  a  large  fly  on  a  piece  of  thin  mica 
surrounded  by  needles  so  that  the  frog  would  be  pricked 
in  its  efforts  to  seize  it.  This  did  not  prevent  the  creature 
from  snapping  at  the  fly  indefinitely.  "In  one  instance 
the  frog,  which  had  been  fasting  for  seventy-two  hours, 
continued  to  snap  at  the  needle  protected  fly  until  it  had 
entirely  skinned  its  upper  jaw.'^  This  convinced  Abbot 
that  "the  wits  of  a  frog  were  too  limited  to  be  demon- 
strated." Knauer  found  that  frogs  would  snap  at  worms  on 
the  other  side  of  a  glass  plate  and  persist  in  doing  so  at  inter- 
vals all  day  without  learning  that  their  attempts  were  futile. 

On  the  other  hand,  several  cases  are  recorded  of  frogs  as 
well  as  toads  being  trained  to  come  for  food.  Toads  often 
occupy  for  a  long  time  a  particular  retreat  to  which  they 
return  after  making  their  nocturnal  tours,  thus  showing 
that  they  have  a  memory  for  location.  Yerkes  in  his  stud- 
ies of  habit  formation  in  frogs  finds  that  frogs  learn  very 
slowly.  In  the  experiments  performed  the  frogs  were  placed 
in  a  long  box  furnished  with  a  tank  filled  with  water  at  one 
end  and  divided  so  that  the  frog  would  have  to  take  a  cer- 
tain course  from  the  point  of  entrance  to  get  into  the  tank. 
The  labyrinth  employed  was  very  simple,  but  it  took  from 
50  to  100  trials  for  a  frog  to  learn  to  avoid  the  closed  pas- 
sages and  to  reach  the  water  by  the  most  direct  route.  The 
associations  once  formed  were  found  to  persist  for  over  a 
month. 

Schaeffer^s  recent  experiments  on  habit  formation  in 
frogs  have  shown  that  frogs  may  learn  to  avoid  disagree- 
able objects  after  a  very  few  trials.  When  offered  hairy 
caterpillars  the  frogs  would  eagerly  snap  at  them  and  then 
quickly  eject  them  from  the  mouth.  After  from  four  to 
seven  such  experiences  the  frogs  came  to  leave  the  cater- 
pillars alone. 


INTELLIGENCE  IN  LOWER  VERTEBRATES    229 

In  another  set  of  experiments  frogs  were  offered  earth- 
worms which  had  been  dipped  in  chemicals.  These  worms 
were  frequently  snapped  at  and  swallowed.  The  diet  pro- 
duced in  some  cases  symptoms  of  uneasiness  and  some  of 
the  frogs  would  avoid  eating  earthworms  for  several  days 
afterward,  although  they  would  partake  freely  of  other 
kinds  of  food. 

An  apparatus  was  arranged  so  that  the  frogs  would  re- 
ceive an  electric  shock  every  time  they  snapped  at  a  worm. 
These  frogs  would  avoid  food  altogether  for  a  few  days 
after  the  shock. '  Whether  an  association  was  formed  in 
this  case,  or  whether  the  result  is  due  simply  to  the  per- 
sistance  of  the  effect  of  the  strong  stimulus  is  uncertain. 
That  the  frogs  learn  to  avoid  certain  kinds  of  food  more 
quickly  than  they  learn  to  follow  a  particular  path  may, 
as  Schaeffer  suggests,  be  due  to  the  fact  that  the  discrim- 
ination of  food  is  so  common  an  experience  in  the  course 
of  a  frog's  daily  life.  The  greater  severity  of  the  penalty 
for  error  may  be  also  a  factor. 

WTiat  has  been  written  on  intelligence  of  reptiles  is  for 
the  most  part  in  the  form  of  scattered,  casual  observations. 
We  have  several  records  of  the  taming  of  different  reptiles, 
of  their  following  their  keepers,  their  distinguishing  between 
different  persons,  and  their  coming  when  called.  Delboeuf 
kept  two  lizards  in  captivity  and  in  time  they  became  quite 
attached  to  their  keeper.  They  would  run  to  him  from 
across  the  room  when  called  and  crawl  upon  his  body  in  the 
hope  of  being  fed.  Each  showed  jealousy  if  any  attention 
were  paid  to  the  other. 

Gilbert  White  in  his  Natural  History  of  Selboume  gives 
an  account  of  a  tortoise  which  distinguished  between  dif- 
ferent persons  and  became  much  attached  to  an  old  lady  so 
that  "whenever  the  good  old  lady  came  in  sight  who  had 


230   INTELLIGENCE  IN  LOWER  VERTEBRATES 

waited  on  it  for  more  than  thirty  years,  it  always  hobbled 
with  awkward  alacrity  toward  its  benefactress,  while  to 
strangers  it  was  altogether  inattentive. '*  Jesse  writes  of  a 
young  alligator  which  followed  its  master  about  the  house 
like  a  dog,  "  scrambling  up  the  stairs  after  him,  and  showing 
much  affection  and  docility/' 

The  formation  of  habits  in  turtles  has  been  studied  by 
Yerkes.  A  simple  labyrinth  was  employed  through  which 
the  turtle  was  left  to  find  its  way.  Fifty  trials  were  made, 
six  or  eight  being  given  each  day,  and  the  time  recorded 
which  the  turtle  required  to  make  its  escape.  The  way  was 
learned  with  a  fair  degree  of  rapidity,  the  time  taken  in 
successive  trips  being  shortened  rapidly  at  first  and  then 
more  slowly. 

In  other  experiments  turtles  learned  not  to  fall  off  a  board 
after  a  number  of  trials.  More  recently  habit  formation 
has  been  studied  in  the  turtle  Chrysemis  by  Cast  eel,  who 
found  that  the  animals  learn  to  discriminate  between  colors 
and  to  distinguish  different  series  of  parallel  lines  of  the 
same  size,  but  with  the  lines  of  different  width.  Learning 
was  slow,  since  on  the  average  "  183  trials  were  necessary  to 
establish  discrimination."  As  in  the  frog  what  was  ac- 
quired was  not  soon  forgotten;  one  specimen  showed  "per- 
fect memory"  for  a  line  pattern  two  weeks  after  it  had 
been  learned. 

In  general  it  may  be  said  that  the  intelligence  of  reptiles 
is  on  a  higher  level  than  in  fishes  and  amphibians.  The 
subject  is  one  upon  which  we  have  little  well  established 
information,  and  it  affords  an  interesting  field  for  future 
investigation. 

BIBLIOGRAPHY 

Bateson,  W.     On   the  Sense   Organs  and  Perceptions  of   Fishes. 
Jour.  Mar.  Biol.  Ass.  United  Kingdom,  1,  225,  '87. 


INTELLIGENCE  IN  LOWER  VERTEBRATES   231 

Casteel,  D.  B.     The  Discriminative  Ability  of  the  Painted  Turtle. 

Jour.  An.  Behavior,  1,  1,  '11. 
Delboeuf,  J.     Quelques  Reflexions  Generales  k  propos  de  la  Psy- 

chologie  des  Lezards.  Rev.  Scient.  4,  805,  '95. 
Edinger,  L.     Haben  die  Fische  ein  Gedachtniss?    Allg.     Zeitung. 

'99.     (Trans,  in  Smithsonian  Rep.,  '99,  375.) 
Goldsmith,  M.     R^cherches  sur  la  Psychologie  de  quelques  Poissons 

Littoraux.     Bull.  Inst.  Gen.  Psych.,  Paris,  5,  51,  '05. 
Gurley,  R.   R.    The   Habits  of  Fishes.    Am.  Jour.  Psych.,  13, 
408,  '02. 
Herrick,  C.  J.    The  Organ  and  Sense  of  Taste  in  Fishes.     BuU. 

U.  S.  Fish  Comm.,  22,  237,  '03. 
Holmes,  S.  J.    The  Biology  of  the  Frog,  N.  Y.  '06. 
Landois,  H.     Haben  die  Fische  Gedachtniss?    Zool.  Garten,  38, 

124,  '97. 
Lecaillon,   a.     Sur  quelques  Faits  Relatifs  k  I'Ethologie  et  la 

Psycho-Physiologie  des   Batraciens.     Bull.  Inst.  G6n.  Psych., 

8,  142,  '08. 
MObius,  K.     Die  Bewegung  der  Thiere  und  ihr  psychischer  Horizont. 

Schrift.  d.  naturwiss.  Ver.  f.  Schleswig-Holstein,  1,  113,  '73. 
Reighard,  J.     An  Experimental  Field-study  of  Warning  Coloration 

in  Coral-reef  Fishes.     Carnegie  Inst.    Pubs.  103,  '09. 
ScHAEFFER,  A.  A.     Habit  Formation  in  Frogs.    Jour.  An.  Behavior, 

1,  309,  '11. 
Thorndike,  E.  L.    a  Note  on  the  Psychology  of  Fishes,  Am.  Nat., 

33,  923,  '99. 
Triplett,  N.  B.    The  Educability  of  the  Perch.  Am.  Jour.  Psych., 

12,  354,  '01. 
Washburn,  M.  F.  and  Bentley,  I.  M.     The  Estabhshment  of  an 

Association  Involving  Color  Discrimination  in  the  Creek  Chub, 

Semotilus  atromaculatus.     Jour.  Comp.  Neur.  Psych.,  16, 113,  '06. 
Yerkes,  R.  M.     The  Instincts,  Habits  and  Reactions  of  the  Frog. 

I.  Associative  Processes  of  the  Green  Frog.     Harvard  Psych. 

Studies,  1,  579,  '03. 

Space  Perceptions  in  Tortoises.    Jour.  Comp.  Neur.  Psych., 

14,  17,  '04. 


CHAPTER  XII 
THE  INTELLIGENCE  OF  MAMMALS 

"Le  premiere  partie  de  cet  ouvrage  d^montre  que  les  betes  sont 
capable  de  quelques  connoissances.  Ce  sentiment  est  celui  du 
vulgaire:  il  n'est  combattu  que  par  des  philosophes,  c'est  k  dire, 
par  des  hommes  qui  d' ordinaire  aiment  mieux  une  absurdity  qu'ils 
imaginent,  qu'une  v^rite  qui  tout  le  monde  adopt." — Condillac, 
Train  des  Animaux. 

"The  power  of  association  over  brutes  is  very  evident  in  all  the 
tricks  which  they  are  taught;  and  the  whole  nature  of  each  brute, 
which  has  been  brought  up  among  others  of  the  same  species,  is  a 
compound  of  instinct,  his  own  observations  and  experiences,  and 
imitation  of  those  of  his  own  species." — Hartley,  Observations  on 
Man. 

"  Animals  pass  from  one  imagination  to  another  by  the  connection 
which  they  have  felt  before;  for  example,  when  his  master  takes  a 
stick,  the  dog  fears  a  whipping.  And  in  many  instances  children 
with  the  rest  of  mankind  proceed  nowise  differently  in  their  passages 
from  thought  to  thought." — Leibnitz,  New  Essays  Concerning 
Human  Understanding. 

Until  quite  recently  most  of  our  knowledge  of  the  psy- 
chology of  mammals,  as  of  other  animals,  was  obtained 
simply  by  watching  them.  In  this  way  has  been  accumu- 
lated a  large  fund  of  information  concerning  their  instincts 
and  habits,  and  to  a  certain  extent  their  general  intelligence. 
But  in  this  as  in  other  fields  of  investigation,  the  method  of 
experiment  has  come  to  be  indispensable  when  the  attempt 
is  made  to  study  the  phenomena  analytically.  There  is  of 
course  no  especial  magic  in  the  experimental  method;  it  is 
simply  a  means  of  improving  the  conditions  of  observation. 
And  in  animal  psychology  especially,  the  method  may  have 
drawbacks  which  counterbalance  some  of  its  advantages. 

232 


THE  INTELLIGENCE  OF  MAMMALS  233 

The  labyrinth  and  puzzle  box  devices  of  our  psychological 
laboratories  have  the  advantage  of  enabling  us  to  study 
animal  behavior  under  strictly  controlled  conditions,  and 
they  readily  yield  results  in  a  form  capable  of  easy  tabula- 
tion, but  they  have  been  criticized  with  a  certain  measure 
of  justice  on  the  ground  that  the  artificial  conditions  to 
which  the  animals  are  exposed  make  them  appear  more 
stupid  than  they  really  are.  A  fox  in  its  wary  prowlings 
for  prey  or  in  its  attempts  to  outwit  its  pursuers  may  mani- 
fest a  considerably  higher  degree  of  intelligence  than  it 
would  show  if  confined  to  a  box  from  which  it  had  to  liber- 
ate itself  by  raising  levers  and  pulling  bolts.  In  wild  an- 
imals especially  there  is  a  falling  off  of  spirit  and  initiative 
when  they  are  placed  in  an  artificial  environment.  The 
lion  of  the  desert  is  a  very  different  creature  from  the  lion 
of  the  circus;  the  keenness  and  alertness  with  which  the  one 
conducts  his  hunt  for  prey,  stand  in  marked  contrast  with 
the  melancholy  and  reluctant  performances  of  the  other 
when  under  the  trainer's  whip.  Other  animals  such  as 
raccoons  and  some  monkeys  take  to  a  life  of  confinement 
much  more  readily,  and  therefore  afford  particulai'ly  valu- 
able subjects  for  experiment.  Those  experiments  are  of 
the  most  value  which  stimulate  to  the  greatest  extent  the 
free  exercise  of  an  animal's  faculties,  and  in  order  to  secure 
this  result  an  animal's  instinctive  interests  and  promptings 
should  be  given,  so  far  as  possible,  free  play. 

The  experiments  of  the  last  few  years  have  given  us  a 
more  just  estimate  of  the  nature  and  limitations  of  the 
intelligence  of  the  higher  animals  than  we  formerly  possessed. 
Animal  psychologists  have  come  to  scrutinize  their  results 
much  more  closely  and  to  be  much  more  cautious  in  their 
statements  regarding  what  goes  on  in  the  mind  of  the  animal 
studied.    The  interpretation  of  what  mental  processes  are 


234  THE  INTELLIGENCE  OF  MAMMALS 

involved  in  most  animal  performances  is  a  matter  of  much 
difficulty.  We  may  be  guided  on  the  one  hand  by  analogy 
with  ourselveS;  which  leads  us  to  infer  that  actions  similar 
to  our  own  are  accompanied  by  similar  mental  states;  and 
by  the  law  of  parsimony  on  the  other,  which  forbids  us  to 
assume  the  existence  of  higher  mental  qualities  if  the  phe- 
nomena can  be  explained  in  terms  of  simpler  mental  pro- 
cesses. In  the  imperfect  state  of  our  knowledge  these  two 
guides  often  lead  to  opposed  conclusions.  If  we  applied  the 
principle  of  Morgan  to  the  psychology  of  our  fellow  human 
beings  we  should  be  continually  led  astray.  So  in  our  inter- 
pretations of  the  psychology  of  the  higher  animals  we  may 
very  frequently  be  "missing  it^'  more  or  less  widely  in  our 
adherence  to  this  principle.  The  antecedent  probability 
in  favor  of  not  giving  the  animal  the  benefit  of  the  doubt 
diminishes  as  we  ascend  the  scale  of  psychic  life.  We  may 
suspect  that  our  interpretations  "fall  short/'  but  our  opin- 
ions cannot  be  said  to  rest  on  a  secure  basis  until  we  are 
able  to  support  them  by  experimental  proof.  The  principle 
of  Morgan  affords  a  check  to  the  natural  tendency  to  "an- 
thropomorphism^' which  is  a  common  human  failing;  it 
throws  the  burden  of  proof  on  whomsoever  attempts  to  es- 
tablish the  existence  in  animals  of  higher  faculties,  and  if 
the  positive  conclusions  to  which  it  permits  us  to  come  fall 
short  of  the  truth  we  can  at  least  rely  on  them  so  far  as 
they  go. 

As  a  typical  instance  of  the  workings  of  the  animal  mind 
we  may  cite  the  performances  of  Professor  Lloyd  Morgan's 
dog,  Toby,  which  had  learned  how  to  open  a  gate  that  led 
out  of  his  master's  yard.  The  gate  was  fastened  by  a  latch, 
but  swung  open  by  itself  if  the  latch  was  raised.  Whenever 
the  dog  desired  to  make  his  escape  he  put  his  head  between 
the  bars,  lifted  the  latch  and  went  out.     Such  an  act-  might 


THE  INTELLIGENCE  OF  MAMMALS  235 

of  course  have  been  the  result  of  the  dog's  studymg  the 
hinges,  latch  and  general  make-up  of  the  gate,  and  conclud- 
ing that  if  the  latch  were  raised  the  gate  would  be  free  to 
swing  open.  Such  a  course  would  be  a  very  natural  one 
for  a  human  being,  but  few  would  consider  that  a  dog 
would  be  likely  to  follow  it.  The  dog  might,  however, 
learn  to  open  the  gate  by  watching  someone  do  it  and  then 
imitating  him.  In  this  case  the  dog  might  be  thought  to 
conclude  that  "since  a  man  lifted  the  latch  and  went  out, 
therefore,  I  can  lift  the  latch  and  go  out."  As  a  matter  of 
fact  the  dog  learned  to  make  his  escape  in  neither  of  these 
ways.  His  method  of  learning  the  trick  which  was  watched 
from  the  beginning  was  as  follows:  Being  placed  in  the 
yard  from  which  he  was  anxious  to  escape  Toby  poked  his 
head  between  the  bars  of  the  fence  in  various  places  and  by 
chance  placed  it  under  the  latch  and  raised  it,  when  the 
gate  swung  open  and  he  scampered  out  on  the  street.  The 
method  pursued  was  one  of  trial  and  error.  The  fortunate 
movement  which  effected  the  dog's  liberation  was  associated 
with  the  perception  of  the  latch,  but  the  association  was  not 
perfect  at  first.  Only  after  ten  or  twelve  experiences,  in 
which  the  number  of  times  the  dog  poked  his  nose  through 
wrong  places  gradually  diminished,  did  he  learn  to  go 
directly  to  the  right  place,  and  raise  the  latch. 

The  experiments  of  Thorndike  have  convinced  him  that 
the  intelligence  of  animals  is  limited  to  the  type  that  we 
have  just  considered.  He  holds  that  animals  do  not  draw 
inferences  and  that,  barring  the  apes,  it  is  doubtful  if  they 
have  ideas.  Thorndike's  experiments  were  among  the 
first  systematic  attempts  to  get  at  the  nature  and  limitation 
of  animal  intelligence  by  means  of  experimental  methods. 
Through  his  pronounced  spirit  of  iconoclasm  toward  anec- 
dotal  psychology  and  anthropomorphism  Thorndike  was 


236 


THE  INTELLIGENCE  OF  MAMMALS 


led  to  adopt  an  extreme  position  which  has  not  been  justi- 
fied by  future  experiments  and  which  was  not  consistent 
with  many  of  his  own  results.  Though  extreme,  the  reac- 
tion against  the  method  of  writers  who  based  their  deduc- 
tions concerning  animal  intelligence  on  stories  and  casual 
observations  was  none  the  less  wholesome,  as  it  succeeded 
in  stimulating  the  study  of  animal  intelligence  by  more 
careful  and  critical  methods  than  those  formerly  employed. 


FiQ.  15. — ^Puzzle  box  used  in  the  experiments  of  Thorndike  on  cats. 
(After  Thorndike.) 

In  most  of  Thorndike^s  experiments  boxes  were  employed 
from  which  animals  could  escape  by  raising  a  lever,  pulling 
a  cord,  or  by  some  such  simple  device.  In  some  cases  vari- 
ous combinations  of  these  devices  were  used.  A  hungry 
cat  or  dog  was  confined  in  the  box  and  food  was  placed  on 
the  outside  so  that  it  could  be  seen.  The  animal  in  its 
efforts  to  get  out  and  obtain  the  food  would  usually  begin 
by  biting  and  clawing  the  bars  of  the  box.  Sooner  or  later 
a  lucky  movement  would  raise  a  lever  or  pull  a  cord  so  that 
the  door  of  the  box  would  open  and  allow  the  animal  to  get 
the  food.    After  this  the  animal  would  be  put  into  the  box 


THE  INTELLIGENCE  OF  MAMMALS  237 

a  second  time,  when  it  usually  recommenced  its  bitings  and 
clawingS;  but  it  generally  effected  its  escape  more  quickly 
than  at  first.  The  experiment  was  then  repeated  until  the 
animal  had  perfectly  mastered  the  means  of  escape.  The 
time  taken  to  escape  was  recorded  and  was  found  to  di- 
minish, as  a  rule,  with  successive  trials. 

In  one  set  of  experiments  a  cat  was  liberated  from  the 
box  whenever  she  licked  herself,  and  in  another  set  when- 
ever she  scratched  herself.  Although  there  could  be  no 
perceptible  relation  between  the  means  employed  and  the 
end  achieved,  the  cats  learned  to  make  the  appropriate 
movement  after  being  put  in  the  box,  although  their  asso- 
ciation curves  showed  a  gradual  descent.  There  is  a  curious 
tendency  for  the  act  to  be  performed  less  and  less  vigorously. 
"The  licking  degenerates  into  a  mere  quick  turn  of  the  head 
with  one  or  two  motions  up  and  down  with  tongue  extended. 
Instead  of  a  hearty  scratch,  the  cat  waves  its  paw  up  and 
down  rapidly  for  an  instant." 

Previous  experience  is  a  factor  which  influences  the  quick- 
ness of  forming  associations.  "After  getting  out  of  six  or 
eight  boxes  by  different  sorts  of  acts  the  cat's  general  tend- 
ency to  claw  at  loose  objects  within  the  box  is  strengthened 
and  its  tendency  to  squeeze  through  holes  and  bite  bars  is 
weakened;  accordingly  it  will  learn  associations  along  the 
general  line  of  the  old  much  more  quickly.  Further,  its 
tendency  to  pay  attention  to  what  it  is  doing  gets  strength- 
ened, and  this  is  something  which  may  properly  be  called 
a  change  in  degree  of  intelligence."  None  of  the  acts  per- 
formed by  cats  and  dogs  in  his  numerous  experiments 
exhibits,  according  to  Thorndike,  any  power  of  reasoning 
and  usually  no  association  of  ideas.  A  point  upon  which 
Thorndike  lays  especial  stress  is  the  gradual  descent  of  the 
curves  representing  the  times  required  in  forming  the  asso- 


238  THE  INTELLIGENCE  OF  MAMMALS 

elation.  If  there  were  any  element  of  inference  involved 
there  ought  to  be,  according  to  him,  a  sudden  vertical  de- 
scent of  the  time  curve.  "  Where  the  act  resulting  from  the 
impulse  is  very  simple,  very  obvious,  and  very  clearly  de- 
fined, a  simple  experience  may  make  the  association  per- 
fect, and  we  may  have  an  abrupt  descent  in  the  time  curve 
without  needing  to  suppose  inference.  But  if  in  a  complex 
act,  a  series  of  acts  or  an  ill-defined  act  one  found  a  sudden 
consummation  in  the  associative  process,  one  might  very 
well  claim  that  reason  was  at  work.  Now,  the  scores  of 
cases  recorded  show  no  such  phenomena.  The  cat  does  not 
look  over  the  situation,  much  less  think  it  over,  and  then 
decide  what  to  do.  It  bursts  out  at  once  into  the  activities 
which  instinct  and  experience  have  settled  on  as  suitable 
reactions  to  the  situation,  ^confinement  wlien  hungry  with 
food  outside.'  It  does  not  ever  in  the  course  of  its  successes 
realize  that  such  an  act  brings  food  and  therefore  decide  to 
do  it  and  thenceforth  do  it  immediately  from  decision 
instead  of  from  impulse.  The  one  impulse,  out  of  many 
accidental  ones,  which  leads  to  pleasure,  becomes  strength- 
ened and  stamped  in  thereby,  and  more  and  more  firmly 
associated  w^ith  the  sense-impression  of  that  box's  interior. 
Accordingly  it  is  sooner  and  sooner  fulfilled.  Futile  im- 
pulses are  gradually  stamped  out.  The  gradual  slope  of 
the  time  curve,  then,  shows  the  absence  of  reasoning.  They 
represent  the  wearing  smooth  of  a  path  in  the  brain,  not  the 
decisions  of  a  rational  consciousness." 

Even  ideas  are  unnecessary,  according  to  Thorndike,  to 
account  for  most  feats  of  animal  intelligence.  The  cat, 
which  after  having  made  a  lucky  movement  and  escaped 
from  the  box  and  got  some  fish,  might  be  supposed  to  asso- 
ciate the  appearance  of  the  mechanism  of  escape  with  the 
idea  of  the  pleasure  resulting  from  eating  the  food.     But, 


THE  INTELLIGENCE  OF  MAMMALS  239 

according  to  Thorndike,  the  learning  of  the  cat  may  be 
accounted  for  more  simply.  The  sight  of  the  means  of 
escape  in  the  box,  instead  of  calling  up  the  idea  of  the  pre- 
viously experienced  good  taste  of  the  fish,  has  become 
associated  simply  with  the  movement  necessary  to  effect  an 
escape.  The  consciousness  involved  in  the  acts  consists 
therefore  of  immediate  sense  impressions  and  the  impulses 
to  action  with  which  they  have  been  associated;  the  animals 
"have  no  images  or  memories  at  all,  no  ideas  to  associate." 

The  learning  of  the  animal,  according  to  this  view,  is  on 
a  level  with  the  semi-conscious  perfection  of  many  of  our 
own  activities,  such  as  catching  a  ball  and  playing  tennis, 
where  certain  perceptions  come  to  call  forth  very  quickly 
the  appropriate  motor  response  without  the  intervention 
of  ideas.  We  do  not  reason  about  our  movements  in  such 
cases  but  the  right  ones  come  to  be  performed  as  the  result 
of  stamping  in  the  movements  that  brought  success.  Few 
of  us  have  paid  particular  attention  to  the  movements  of 
the  tongue  in  chewing  food,  but  a  little  observation  directed 
to  the  subject  cannot  fail  to  impress  one  with  the  deftness 
with  which  this  member,  otherwise  so  unruly,  avoids  being 
bitten,  and  with  the  efficient  way  in  which  it  helps  to  masti- 
cate the  food.  The  tongue  probably  performs  many  of  its 
movements  instinctively,  but  it  has  doubtless  learned  a  good 
deal  by  experience,  without  being  consciously  directed. 

Thorndike's  theory  of  the  nature  of  animal  intelligence 
has  the  principle  of  parsimony  in  its  favor,  but  its  author 
has  endeavored  to  give  it  further  support  by  a  number  of 
experiments  designed  to  test  the  presence  of  ideas  in  the 
animal's  mind.  One  of  these  was  as  follows:  A  cat  was 
made  to  go  through  a  door  into  a  box  where  she  was  shut  in. 
By  pulling  a  loop  the  cat  gets  out  and  eats  fish.  After  being 
put  in  through  the  door  a  number  of  times  and  fed  when- 


240  THE  INTELLIGENCE  OF  MAMMALS 

ever  she  made  her  escape,  the  cat  came  to  enter  the  box  of 
her  own  accord.  Does  she  associate  the  idea  of  being  in 
with  the  idea  of  eating  fish  and  enter  accordingly?  Thorn- 
dike  endeavored  to  answer  this  question  by  dropping  cats 
into  the  box  through  a  door  in  the  top,  and  then  feeding 
them  as  before  when  they  got  out.  The  cats  had  the  same 
opportunity  of  associating  the  idea  of  being  in  the  box  with 
the  idea  of  eating  fish,  but  the  element  lacking  was  the 
impulse  to  walk  in  through  the  door.  All  of  these  cats, 
three  in  number,  failed  to  enter  the  box  after  fifty,  sixty 
and  seventy-five  trials  respectively.  "Either  a  cat  cannot 
connect  ideas,  representations,  at  all,"  says  Thorndike, 
*^or  she  has  not  the  power  of  progressing  from  the  thought  of 
being  in  to  the  act  of  going  in.  .  .  .  The  impulse  is  the  sine 
qua  non  of  the  association.  The  second  cat  has  everything 
else,  but  cannot  supply  that." 

In  several  other  experiments  cats  and  dogs  were  placed 
in  a  box,  and  the  experimenter  would  take  the  paw  of  the 
animal  and  make  the  movement  necessary  to  open  the  box, 
after  which  the  animal  would  be  allowed  to  go  out  and  get 
food.  This  was  repeated  ten  or  fifteen  times  and  then  the 
animal  was  left  to  its  own  devices.  After  numerous  ex- 
periments of  this  sort  with  various  kinds  of  boxes  it  was 
found  that  the  animals  uniformly  failed  to  profit  by  this 
mode  of  instruction.  None  of  the  animals  which  failed  to 
get  out  of  a  box  of  their  own  accord  succeeded  in  escaping 
after  having  been  several  times  put  through  the  necessary 
movements.  They  had  the  opportunity  to  associate  the 
idea  of  certain  movements  with  escaping  and  getting  food, 
at  least  provided  they  paid  attention  to  what  was  being 
done  with  them.  But  the  animaPs  own  impulse  to  do  the 
act  was  lacking.  ''The  animal  cannot  form  an  association 
leading  to  an  act  unless  the  particular  impulse  to  that  act 


THE  INTELLIGENCE  OF  MAMMALS  241 

is  present  as  an  element  of  the  association;  he  cannot  supply 
it  from  a  general  stock.  The  groundwork  of  animal  asso- 
ciations is  not  the  association  of  ideas,  but  the  association 
of  idea  or  sense-impression  with  impulse.'* 

Notwithstanding  his  general  attitude  of  opposition  to  the  ^ 
doctrine  of  association  of  ideas  in  animals,  Thorndike  re- 
cords a  few  experiments  which  led  him  to  the  conclusion 
that  in  certain  cases  such  associations  may  occur.  In  one 
case  a  hungry  cat  was  placed  in  a  box  and  the  experimenter 
sat  about  eight  feet  away  from  it.  At  intervals  of  about 
two  minutes  he  would  say,  "I  must  feed  those  cats."  Ten 
seconds  afterward  he  would  take  a  piece  of  fish,  go  to  the 
box  and  hold  it  so  that  the  cat  was  compelled  to  climb  up 
the  front  of  the  box  to  obtain  it.  Would  the  cat  after  a 
number  of  trials  come  to  associate  (A)  the  sound  of  the 
words  with  (B)  the  sense  impression  of  the  experimenter's 
movements  in  taking  the  fish  and  walking  to  the  box,  and 
climb  up  (C)  before  it  had  experienced  the  second  term  (B) 
of  the  association?  If  so  Thorndike  concludes  that  the 
action  of  the  cat  "  is  to  be  explained  by  the  presence  through 
association,  of  the  idea  (B)."  The  possibility  is  left  open 
"that  (A)  was  associated  directly  with  the  impulse  to  (C), 
although  that  impulse  was  removed  from  it  by  ten  seconds 
of  time."  But  Thorndike  thinks  this  is  ''  highly  improbable, 
unless  the  neurosis  of  (A),  and  with  it  the  psychosis,  con- 
tinues until  the  impulse  to  (C)  appears.  But  if  it  does  so 
continue  during  the  ten  seconds,  and  thus  get  directly  linked 
to  (C),  we  have  exactly  a  representation,  an  image,  a  memory 
in  the  mind  for  eight  or  ten  seconds."  Leaving  out  of  ques- 
tion the  existence  of  an  idea  of  (A),  during  the  interval, 
why  should  we  say  that  it  is  "highly  improbable"  that 
(A)  should  become  directly  linked  with  (C)?  The  implica- 
tion of  the  argument  is  that  the  time  separating  the  two 


242  THE  INTELLIGENCE  OF  MAMMALS 

events  is  too  great.  But  if  (A)  can  be  associated  with  (B) 
over  an  interval  of  ten  seconds  why  can  it  not  be  directly 
associated  with  the  impulse  (C)  over  an  interval  but  slightly 
greater?  Ten  seconds  after  (A)  the  cat  sees  (B),  eleven 
seconds  after  she  performs  (C).  That  the  cat  had  pre- 
viously associated  (B)  and  (C)  does  not  necessarily  play 
any  part  in  the  process.  The  experiment  proves  only  that 
some  sort  of  a  neurosis  persists  from  (A)  during  ten  or  more 
seconds  of  time;  but  it  fails  to  afford  any  proof  of  the  r6le  of 
the  idea  (B)  in  the  process.  It  shows  the  persistence  of 
impressions,  not  the  association  of  ideas.  Indirectly  it 
may  support  the  theory  of  association.  If  the  neurosis  of 
(A)  persists  and  is  accompanied  by  consciousness  in  the 
form  of  an  idea  of  (A),  ideas  of  (B)  probably  occur  also,  and 
if  ideas  occur  why  may  they  not  become  associated  as  well 
as  impressions  and  impulses?  Such  ideas  may  be  more  like 
after-images  in  ourselves,  but  no  sharp  line  can  be  drawn 
between  the  latter  and  ideas  properly  so  called.  While 
such  an  experiment  as  the  one  described  may  not  prove  the 
association  of  ideas  it  may  serve  to  make  association  more 
probable  by  showing  the  persistence  of  impressions. 

The  experiments  of  L.  W.  Cole  on  the  raccoon  yielded 
better  evidence  of  the  existence  of  ideas  than  the  investiga- 
tions of  Thorndike,  owing  perhaps  to  the  greater  degree  of 
intelligence  of  the  animals  employed.  Raccoons  learned  to 
get  out  of  a  box  with  seven  fastenings  consisting  of  two 
buttons,  two  loops,  a  thumb  latch,  a  treadle  and  a  hook. 
They  were  first  put  into  boxes  with  one  or  two  of  these 
devices,  and  when  these  were  learned  others  were  added  until 
the  above  combination  was  reached,  which  seemed  to  be 
about  the  limit  of  a  raccoon^s  learning  capacity.  The  rac- 
coons in  attacking  the  fastenings  did  not  take  them  up  in 
any  constant  order  in  successive  trials,  but  they  showed  a 


THE  INTELLIGENCE  OF  MAMMALS  243 

good  memory  for  any  combination  they  may  have  learned. 
Three  raccoons  which  had  learned  to  open  the  box  with 
seven  fastenings  were  put  into  the  box  again  after  an  interval 
of  147  days.  Only  one  individual  succeeded  in  undoing  all 
of  the  seven  fastenings  and  escaping.  His  times  in  four 
successive  trials  were  34,  28,  131,  and  182  seconds.  The 
other  two  worked  most  of  the  fastenings,  but  generally 
failed  to  undo  a  horizontal  lock.  Da\ds  found  that  rac- 
coons remembered  how  to  undo  the  fastenings  of  a  puzzle 
box  for  over  a  year. 

One  significant  feature  of  the  raccoons'  method  of  attack 
on  their  problems  is  that  they  employ  different  means  of 
accomplishing  the  same  result.  They  worked  fastenings 
with  either  the  right  or  the  left  paw  or  with  both  together. 
"All  of  the  raccoons  turned  a  button  once  or  twice  with 
the  nose  in  early  trials,  then  settled  down  to  w^orking  it  with 
the  paw."  This  looks  very  much  as  if  there  were  something 
besides  the  sensori-motor  associations  assumed  by  Thorn- 
dike.  We  cannot  say  that  there  is  nothing  but  the  asso- 
ciation of  the  sight  of  a  certain  object  with  a  particular 
impulse  to  movement,  if  in  effecting  a  certain  change  an 
entirely  different  organ  is  substituted  for  the  one  pre- 
viously employed.  When  an  animal  moves  with  its  paw  a 
fastening  it  formerly  moved  with  its  nose  it  gives  evidence 
of  being  guided  by  an  idea  of  what  it  is  setting  out  to 
accomplish. 

Evidence  for  the  existence  of  ideas  was  derived  also  from 
other  sources.  It  was  found  that  raccoons  were  able  to 
associate  being  in  a  box  with  getting  food  after  they  came 
out,  so  that  when  they  were  dropped  in,  as  in  the  experi- 
ment with  the  cats,  they  came  after  a  while  to  go  into  the 
box  of  their  own  accord.  The  motor  impulse  to  enter  the 
box  was  not  in  these  cases  an  element  in  the  association 


244  THE  INTELLIGENCE  OF  MAMMALS 

that  was  formed.  Apparently  the  thought  of  bemg  in  the 
box  made  the  raccoons  go  in. 

Cole  found  that  the  raccoons  were  aided  in  learning  the 
mechanism  of  escape  from  the  box  if  they  were  put  through 
the  appropriate  movements.  Not  only  was  the  average 
time  required  for  such  animals  to  escape  from  the  box  short- 
ened as  compared  with  the  time  required  by  the  individuals 
which  were  unaided,  but  several  animals  which  failed  entirely 
to  make  their  escape  succeeded  in  getting  out  after  having 
been  several  times  put  through  the  act.  This  result  may 
be  indicative  of  the  existence  of  ideas,  but  not  entirely  con- 
clusive, as  it  might  be  explained  possibly  as  the  result  of  the 
animaFs  attention  having  been  limited  to  a  certain  part  of 
the  box.  This  might  direct  his  efforts  toward  the  spot 
where  he  would  be  more  likely  to  hit  upon  the  fortunate 
movement.  Raccoons  which  had  come  to  climb  upon  a 
box  and  enter  through  a  hole  in  the  top  after  having  several 
times  dropped  through  the  hole  would  dodge  in  through  the 
door  if  it  were  left  open  after  their  exit.  As  in  the  cases 
where  the  raccoons  learned  to  escape  by  themselves,  they 
used  different  paws  to  undo  various  fastenings,  although 
one  fore  paw  was  almost  always  used  when  the  experimenter 
guided  their  actions.  Whether  or  not  the  raccoon  re- 
peats the  action  he  is  put  through  depends  much  upon  his 
convenience.  If  it  is  the  easiest  way  he  will  continue  as  he 
was  taught;  if  not  he  is  apt  to  substitute  some  other  method. 

If  a  raccoon  has  learned  to  undo  a  latch,  turn  a  button,  or 
pull  a  loop,  he  attacks  the  same  object  when  it  is  placed  in  ^ 
different  part  of  the  enclosure.  Davis,  however,  attained 
a  different  result  with  some  of  his  raccoons,  which  would 
often  spend  considerable  time  in  vain  clawing  about  where 
the  fastening  had  been  placed.  Probably  these  different 
results  depend  upon  differences  in  the  previous  experiences 


THE  INTELLIGENCE  OF  MAMMALS  245 

of  the  animals,  or  their  differences  in  temperament  and 
general  intelligence,  which  Davis  finds  are  very  marked. 
In  one  case  a  loop  was  left  lying  upon  the  top  of  the  box. 
When  seen  by  the  raccoon  it  was  clawed  back  into  the  box 
and  then  pulled.  Is  this  merely  the  blind  association  of 
perception  with  motor  impulse?    I  think  not. 

Cole  found  that  if  food  was  put  in  a  box  and  the  place 
where  the  raccoon  had  formed  the  habit  of  entering  was 
closed,  the  animal  would  attempt  to  enter  the  box  at  other 
places.  In  one  experiment  a  tight  box  was  used  with  a 
hole  through  the  top.  After  the  raccoon  had  entered 
through  the  hole  and  obtained  food  a  number  of  times  the 
hole  was  closed  and  an  opening  made  under  one  side  of  the 
box.  At  first  the  animal  attempted  to  enter  in  the  usual 
way,  but  finding  his  passage  barred  soon  left  it;  after  a 
second  attempt  he  discovered  the  side  opening,  entered  and 
secured  the  food.  The  side  opening  was  then  closed  and  a 
wire  cylinder  eighteen  inches  high  placed  on  end  over  the 
original  opening  at  the  top  of  the  box.  Thirty  seconds 
after  the  raccoon  was  released  he  had  climbed  up  the  outside 
and  down  the  inside  of  the  cylinder  and  entered  the  hole. 
The  piece  of  apple  in  the  box  could  not  be  seen,  nor  smelled, 
according  to  Cole,  for  "  the  room  was  full  of  the  odor  of  apple.'' 
The  animal  apparently  retained  an  image  of  the  apple  in  the 
box  and  realized  that  if  he  could  not  reach  it  in  one  way  he 
might  in  another. 

Such  behavior,  in  which  an  animal's  activity  seems  to  be 
directed  to  achieving  a  certain  end,  is  not  uncommon.  A 
case  described  by  Hobhouse  is  suggestive.  A  dog  was 
held  at  the  back  of  a  house  with  which  he  was  unfamiliar 
and  saw  his  master  enter  by  the  back  door  and  appear  at  a 
window  in  the  same  side  of  the  house.  "After  trying  to 
follow  his  master  through  the  back —  unsuccessfully,  because 


246  THE  INTELLIGENCE  OF  MAMMALS 

the  door  is  shut —  he  makes  off  round  two  corners  to  the 
front  door,  and  so  into  the  dining-room.  He  had  never 
been  in  this  room  before,  but  has  once  been  from  the  back 
into  the  house  by  the  front  door.  The  experiment  is  once 
repeated,  and  the  dog  remembers  this  route  five  days  later. 
On  arriving  at  the  house  on  this  occasion,  he  is  taken  through 
a  side  door  into  the  dining-room,  and  then  out  at  the  back. 
He  first  finds  his  way  in  through  the  front  as  mentioned,  and 
then  for  a  further  trial  both  front  and  back  door  are  shut. 
The  dog  goes  to  and  fro  from  one  door  to  the  other,  and  then 
suddenly  goes  right  off  around  the  house,  and  in  by  the  side 
door —  a  route  which  he  had  never  taken  before.  There  may 
have  been  an  element  of  chance  in  this  success,  but,  on  the 
whole  we  seem  to  have  a  series  of  acts  dictated  by  the  desire 
to  find  the  master  operating  on  the  remembrance  of  the 
modes  of  entrance.'^ 

The  ability  of  animals  such  as  horses  to  find  their  way 
back  for  miles  over  a  road  which  they  have  only  followed 
once  is  indicative  of  something  more  than  mere  sensori- 
motor association.  I  well  remember  a  horse  we  once 
owned  whose  memory  for  the  proper  turns  in  the  road  he 
had  taken  in  going  away  from  home  I  had  often  tested  and 
found  to  be  almost  infallible.  Like  many  other  horses  he 
was  a  much  more  willing  traveller  when  homeward  bound, 
but  whether  he  was  influenced  by  an  idea  of  hay  and  oats 
and  rest  to  be  enjoyed  at  the  end  of  his  journey  it  might  be 
hazardous  to  say.  If  the  homing  of  the  animal  were  due 
to  a  blind  sensori-motor  association,  we  should  have  to 
assume  that  the  sight  of  particular  objects  along  his  course 
came  to  be  associated  with  particular  movements;  object 
A,  for  instance,  with  a  slight  turn  to  the  right,  and  object 
B  with  a  slight  turn  to  the  left,  and  so  on.  If  the  animal 
passed  over  the  road  again  in  the  same  direction  we  might 


THE  INTELLIGENCE  OF  MAMMALS  247 

suppose  that  these  various  objects,  by  association,  called 
forth  muscular  impulses  similar  to  those  originally  given, 
thereby  causing  the  animal  to  keep  the  same  path  as  before. 
But  on  the  return  the  objects  are  not  only  met  with  in  a 
different  order,  the  visual  fields  of  the  animal  present  in 
many  respects  a  quite  different  aspect,  and  the  muscular 
impulses  given  at  the  various  turns  are  just  the  reverse  of 
those  given  as  various  landmarks  were  encountered  during 
the  outgoing  journey.  Must  we  not  assume  that  in  the 
animal's  mind,  as  in  our  own,  there  is  a  consciousness  of  the 
general  space  relation  of  objects  seen  along  the  journey  and 
the  animaFs  own  changes  of  position  in  relation  to  those 
objects?  Does  not  the  horse  feel  that  he  is  going  away 
farther  and  farther  from  his  bam,  although  the  latter  cannot 
be  seen,  and  does  he  not  in  some  way  keep  the  general 
topographical  situation  in  mind  amid  all  the  panoramic 
scene-shif tings  in  his  field  of  vision?  If  the  horse  does  not 
have  clearly  defined  ideas  he  seems  to  possess  some  con- 
sciousness of  unperceived  objects  and  their  relation  to 
present  ones.  Horses  and  other  animals  commonly  find 
their  way  home  over  routes  very  different  from  the  ones  on 
which  they  set  out,  even  in  places,  such  as  woods,  where  they 
cannot  perceive  the  general  landscape.  We  may  say  of 
course  that  the  animal  is  guided  by  a  "sense  of  direction,'' 
but  if  we  ascribe  this  faculty  to  the  animal  in  any  but  a 
mystical  sense,  we  can  scarcely  escape  assuming  that  there  is 
something  that  fulfils  the  function  of  a  representative  con- 
sciousness of  the  absent  elements  of  the  situation.  The 
animal  which  makes  for  home  by  a  new  route  cannot  be  said 
to  be  guided  by  sensori-motor  impulses  in  relation  to  certain 
trees  or  rocks,  because  these  objects  have  not  been  asso- 
ciated with  any  impulses  in  the  animaPs  experience.  If  we 
examine  our  own  mental  experience  in  such  a  situation  it 


248  THE  INTELLIGENCE  OF  MAMMALS 

will  be  evident  that  we  have  a  consciousness  of  an  outlying 
region  beyond  our  immediate  sphere  of  perception  which 
stands  in  a  definite  relation  to  the  latter,  and  that  the  object 
of  our  search  lies  in  a  certain  direction.  When  the  per- 
ceptual and  the  ideal  content  of  our  minds  get  disjoined  we 
have  the  horrible  consciousness  of  being  lost.  If  the  ideal 
content  were  absent  and  we  had  nothing  to  depend  upon 
but  a  store  of  sensori-motor  associations  would  we  be  able 
to  get  back  to  our  starting  place?  I  think  not.  And  with- 
out this  content  I  fancy  the  horse  would  be  as  helpless  as 
ourselves. 

Whether  animals  draw  inferences  of  a  simple  sort  is  a 
subject  we  shall  dwell  upon  further  in  the  chapter  on  the 
mental  life  of  apes  and  monkeys.  It  is  not  at  all  likely 
that  animals  have  any  power  of  abstract  or  conceptual 
reasoning;  as  Morgan  remarks  they  probably  "do  not  think 
the  therefore,"  but  mental  action  essentially  inferential  in 
character  may  not  involve  any  processes  of  a  complicated 
kind.  As  Binet  has  attempted  to  make  clear  in  his  work 
on  the  Psychology  of  Reasoning,  there  is  a  fundamental 
similarity  between  reason  and  simple  perception.  The 
shape  and  color  of  an  orange  recall  the  sensations  of  odor, 
taste,  touch  and  muscular  movements  which  we  have  ex- 
perienced in  connection  with  such  visual  impressions  in  the 
past.  These  various  states  are  combined  in  a  percept 
which  seems  to  us  a  simple  object.  The  visual  impression 
has  assimilated  various  other  attributes,  and  we  therefore 
tend  to  act  differently  toward  such  appearances  on  the 
basis  of  this  association.  The  sweet  taste  may  cause  us  to 
reach  out  for  the  orange  and  we  might  justify  our  procedure 
by  a  process  of  reasoning  about  the  relation  of  the  various 
attributes  of  the  object.  But  we  certainly  do  not  do  so 
before  the  sweetness  of  the  orange  is  borne  in  upon  us. 


THE  INTELLIGENCE  OF  MAMMALS  249 

The  sweetness  occurs  to  us  very  quickly  and  directly,  and 
with  it,  if  we  are  hungry,  the  impulse  to  seize,  and  eat  the 
orange.  In  this,  as  in  most  affairs  of  life,  we  arrive  at  the 
conclusion  first  and  reason  about  it  afterward.  Our  con- 
scious reasoning  is  a  process  of  reviewing  and  verification 
which  can  usually  be  dispensed  with,  rather  than  one  of 
discovery.  The  animal  that  reaches  for  an  object  which  it 
has  learned  is  good  to  eat  gets  along  without  the  retrospec- 
tive review.  He  may  go  through  with  mental  processes  of 
various  degrees  of  complexity.  The  visual  sensation  may 
call  up  directly  the  impulse  to  seize  and  eat  the  orange.  It 
may  call  up  along  with  the  latter  the  taste  and  other  attri- 
butes of  the  orange.  It  may  caU  up  the  taste  and  other 
attributes  which  in  turn  arouse  the  impulses  to  seize  and 
eat.  The  various  mental  steps  may  be  present  in  different 
degrees  of  vividness.  The  relation  of  different  states  may 
be  attended  to;  the  animal  may  finally  come  to  "think  the 
therefore, '*  and  so  on. 

"Inference,"  says  Hobhouse,  "is  one  function,  from  the 
simplest  case  quoted  by  Mr.  Morgan  of  the  chick,  up  to  the 
highest  elaboration  of  experience  by  the  human  intellect. 
The  differences  are  differences  in  articulateness  on  the  one 
side,  and  comprehensiveness  on  the  other."  There  is  good 
reason  to  believe  that  animals  profit  by  the  association  of 
ideas,  and  that  they  do  certain  acts,  not  for  their  own  sake, 
but  as  a  means  to  an  ulterior  end  which  is  kept  in  mind. 
If  we  do  not  choose  to  designate  the  mental  operations 
involved  in  such  behavior  by  the  term  reason  we  must  at 
least  admit  that  they  are  on  the  road  to  it. 

How  far  along  animals  like  dogs,  raccoons  and  elephants 
may  be  on  the  highway  toward  reason  properly  so-called 
it  is  impossible  at  present  to  state.  I  have  read  critically 
a  good  many  stories  of  animal  intelligence  which  have  left 


250  THE  INTELLIGENCE  OF  MAMMALS 

in  my  mind  a  general  conviction  that  animals  are  capable 
of  drawing  simple  conclusions,  but  there  are  alternate  pos- 
sibilities of  interpretation  in  many  cases,  and  incompleteness 
of  data  in  so  many  others  that  I  am  unable  to  present  any 
number  of  instances  in  which  inference  of  a  very  articulate 
kind  is  indubitably  shown.  The  subject  is  one  on  which 
we  need  more  experiments  performed  by  investigators 
acquainted  with  the  animal's  previous  history  and  keenly 
alive  to  the  various  possible  psychological  interpretations 
which  may  be  put  upon  an  animal's  behavior.  The  diffi- 
culties and  pitfalls  of  the  subject  are  far  beyond  the  realiza- 
tion of  most  of  the  contributors  to  our  data  on  comparative 
psychology.  There  is  a  large  amount  of  material  too  care- 
fully recorded  to  be  cavalierly  rejected  as  worthless,  but  too 
incomplete  to  be  accepted  as  entirely  conclusive  on  the  sub- 
ject of  animal  inference;  it  will  doubtless  prove  of  great 
value  in  suggesting  lines  for  future  work. 

A  case  in  point  is  the  following  account  of  two  dogs, 
contributed  by  Mr.  Stone  to  Romanes'  "Animal  Intelli- 
gence." "  One  of  them,  the  larger,  had  a  bone,  and  when  he 
had  left  it  the  smaller  dog  went  to  take  it,  the  larger  one 
growled,  and  the  other  retired  to  a  corner.  Shortly  after- 
ward the  larger  dog  went  out,  but  the  other  did  not  appear 
to  notice  this,  and  at  any  rate  did  not  move.  A  few  minutes 
later  the  large  dog  was  heard  to  bark  out  of  doors;  the  little 
dog  then,  without  a  moment's  hesitation,  went  straight  to 
the  bone  and  took  it.  It  thus  appears  evident  that  she 
reasoned — 'That  dog  is  barking  out  of  doors,  therefore  he 
is  not  in  this  room,  therefore  it  is  safe  for  me  to  take  the 
bone.'  The  action  was  so  rapid  as  to  be  clearly  a  conse- 
quence of  the  other  dog's  barking." 

The  behavior  described  will  not  appear  to  anyone  familiar 
with  dogs  as  anything  improbable.    The  doubtful  feature 


THE  INTELLIGENCE  OF  MAMMALS  251 

is  the  syllogism  in  the  dog's  mind.  The  dog  probably  did 
not  reason  the  matter  out  in  the  explicit  fashion  indicated 
by  the  words  employed  by  Mr.  Stone,  and  perhaps  the  writer 
would  not  insist  that  he  did.  The  bark  may  have  made  the 
small  dog  aware  that  the  large  dog  was  out  of  the  room  and 
that  it  was  safe  for  him  to  seize  the  bone.  He  may  not  have 
thought  that  therefore  he  could  safely  seize  the  bone,  but 
the  large  dog  being  away  the  small  one  just  went  after  the 
bone  because  he  was  no  longer  restrained.  It  is  possible, 
though  less  likely,  that  the  bark  simply  served  to  direct  the 
attention  of  the  small  dog  from  whatever  object  it  was 
bestowed  upon,  and  that  it  was  then  directed  to  the  bone 
which  was  seized  because  the  large  dog  was  out  of  the  way. 
Or  it  may  be  that  when  the  small  dog  betook  himself  to  the 
corner  he  fell  half  asleep  and  was  brought  to  himself  by  the 
bark  of  the  other  dog.  How  varied  are  the  interpretations 
that  can  be  made  of  the  contents  of  the  dog^s  mind!  We 
may  feel  convinced  from  our  general  knowledge  of  dog 
behavior  and  the  special  circumstances  of  the  case  that 
there  was  something  in  the  small  dog's  mind  corresponding 
to  ''large  dog  outside,  I  can  now  get  the  bone;"  but  our  con- 
viction does  not  constitute  proof.  And  so  it  goes  with  story 
after  story. 

While  the  proof  of  the  existence  of  explicit  inference  may 
be  difficult  though  by  no  means  impossible,  there  is  a  singular 
lack  of  conclusiveness  in  the  arguments  sometimes  employed 
to  prove  its  absence.  It  is  argued  by  Thomdike  that  the 
gradual  descent  of  the  time  curves  of  learning  in  his  experi- 
ments showed  the  absence  of  reasoning.  But  when  we 
examine  these  curves  it  becomes  apparent  that  their  shape, 
in  a  considerable  proportion  of  cases,  is  far  from  gradual, 
as  in  most  of  the  figures  on  pp.  18,  20  and  24.  Small  finds 
in  the  curves  of  learning  of  the  rat  that  there  is,  as  a  rule,  a 


252  THE  INTELLIGENCE  OF  MAMMALS 

marked  drop  after  the  first  success,  and  a  similar  phenom- 
enon is  remarked  by  Hobhouse.  Still  less  indicative  of  the 
gradual  stamping  in  process  are  the  results  of  the  experi- 
ments of  Davis  and  L.  W.  Cole  on  the  raccoon.  Davis 
found  that  the  time  curves  of  two  children  in  learning  the 
fastenings  of  a  box  were  very  similar  to  those  of  the  raccoon, 
and  Lindley  in  his  Study  of  Puzzles  obtains  very  similar 
results  wdth  young  children. 

When  Thorndike  speaks  of  reasoning  he  evidently  has  in 
mind  a  fairly  typical  process  of  ratiocination,  and  not  the 
simple  process  of  the  inferential  type  described  above, 
which  we  should  most  naturally  expect  in  the  animal  mind. 
A  man  would  find  his  way  out  of  a  puzzle  box  after  once 
discovering  the  proper  method,  but  granting  that  a  cat  has 
a  certain  power  of  inference  it  would  not  be  surprising  that, 
with  her  hazy  consciousness  of  the  situation,  feeble  and  fluc- 
tuating attention  and  indefinite  memory  of  just  what  she 
did  before,  several  trials  would  be  required  to  enable  her 
to  solve  her  problem.  It  is  quite  easy  to  convict  a  cat  of 
Btupidity  by  showing  that  she  cannot  reason  in  human 
fashion.  Whether  she  is  capable  of  mental  action  of  a 
primitive  inferential  type  is  a  quite  different  question. 

We  are  apt  to  overestimate  the  importance  of  the  ability 
to  reason  as  if  it  were  the  chief  thing  of  value  in  intelligent 
behavior.  There  are  other  mental  traits  which  may  enable 
an  animal  to  get  what  it  wants  better  than  an  increment  of 
reasoning  power.  General  activity,  power  of  attention, 
interest,  quickness  of  forming  associations,  delicacy  of  dis- 
crimination, duration  of  memory  and  the  ability  to  form 
complex  associations  are  all  of  the  utmost  importance  in 
many  situations  in  an  animal's  life.  We  frequently  meet 
people  who,  when  they  are  compelled  to  exercise  their  reason- 
ing powers,  act  as  if  the  effort  were  a  painful  one  and  as  if 


THE  INTELLIGENCE  OF  MAMMALS  253 

they  were  groping  in  a  sort  of  intellectual  fog.  We  are  often 
amazed  at  the  obvious  fallacies  they  fall  into,  like  Mrs. 
Eddy's  famous  method  of  proving  propositions  by  inversion; 
but  the  same  people  may  exhibit  an  unusual  degree  of 
keenness  and  practical  sense  in  the  ordinary  affairs  of  life. 
In  animals  these  intellectual  faculties  of  a  lower  order  are 
developed  in  different  ways  according  to  the  habits  of  the 
species.  Give  a  fox  greater  power  of  inferential  thinking, 
but  decrease  his  alertness,  curiosity,  suspiciousness,  and 
quickness  of  perception,  and  he  might  fall  a  victim  to  the 
hunter  while  his  mind  was  occupied  on  some  other  subject. 
In  the  development  of  these  various  intellectual  qualities  \ 
there  has  been  an  enormous  progress  from  the  stupid  mar-  \ 
supials  to  the  apes  and  monkeys,  during  which  the  founda- 
tions for  the  superstructure  of  reason  were  broadly  laid. 
Just  as  the  various  non-intelligent  modifications  of  behavior 
facilitate  the  development  of  intelligence,  so  do  the  diverse  ; 
manifestations  of  intelligence  prepare  the  way  for  the  advent 
of  reason. 

The  question  as  to  whether  animals  imitate  acts  from  which 
they  see  that  other  animals  derive  an  advantage  has  an 
important  bearing  on  our  views  of  their  psychic  development. 
The  word  imitation  is  employed  in  a  very  wide  sense  by 
some  writers  such  as  Tarde  and  Baldwin,  but  it  is  more 
commonly  used  to  designate  the  performance  of  an  act  after 
perceiving  the  act  performed  by  another  creature.  In 
imitation  of  this  type  we  usually  distinguish  two  kinds, 
the  instinctive,  and  the  reflective  or  rational.  In  the  first 
the  perception  of  another  animal  performing  an  act  forms 
the  stimulus  which  sets  off  an  innate  tendency  to  perform 
a  similar  act.  In  fishes  which  run  in  schools  the  turning 
about  of  one  individual  may  cause  all  the  others  to  turn; 
each  individual   has   an   innate   proclivity   to   follow   the 


254  THE  INTELLIGENCE  OF  MAMMALS 

movements  of  the  others,  and  by  vu*tue  of  this  trait  the 
fishes  keep  together  and  escape  common  dangers.  A 
similar  kind  of  imitation  is  met  with  even  in  insects.  Ants, 
according  to  Wasmann,  not  infrequently  imitate  one  an- 
other's acts,  and  other  observers  have  remarked  upon  the 
same  proclivity  in  bees  and  termites. 

Imitation  plays  an  especially  important  role  in  the  life 
of  birds.  According  to  Lloyd  Morgan,  "If  one  of  a  group 
of  chicks  learn  by  casual  experience  to  drink  from  a  tin  of 
of  water,  others  will  run  and  peck  at  the  water,  and  thus 
learn  to  drink.  A  hen  teaches  her  little  ones  to  pick  up 
grain  and  other  food  by  pecking  on  the  ground  and  dropping 
suitable  materials  before  them,  while  they  seemingly  imitate 
her  action  in  seizing  the  grain.  One  may  make  chicks  and 
pheasants  peck  by  simulating  the  action  of  a  hen  with  a 
pencil  point  or  pair  of  fine  forceps.  According  to  Mr. 
Peal  the  Assamese  find  that  the  young  jungle  pheasants  will 
perish  if  their  pecking  responses  are  not  thus  artifically 
stimulated;  and  Professor  Clay  pole  tells  me  that  this  is 
also  the  case  with  young  ostriches  hatched  in  an  incubator.'' 
Chicks  avoid  objects  which  they  perceive  arouse  alarm  in 
others,  and  if  they  see  other  chicks  pecking  at  things  of 
which  they  stand  a  little  in  awe  they  frequently  muster  up 
courage  and  follow  their  companions. 

Birds  learn  to  fear  certain  enemies,  such  as  hawks,  at 
least  in  part,  through  their  instinctive  response  to  the  signs 
of  alarm  in  other  birds.  Their  instinct  of  following  guides 
them  in  their  migration  routes,  in  which,  despite  the  con- 
tention of  Herr  Gatke,  their  course  is  in  all  probability  a 
matter  of  tradition. 

In  contrast  to  the  above  cases  which  may  be  regarded  as 
instinctive  responses  to  particular  stimuli  are  those  instances 
in  which  an  animal  more  or  less  deliberately  copies  the  actions 


THE  INTELLIGENCE  OF  MAMMALS  255 

of  another  with  the  end  of  securing  some  advantage  which 
the  other  animal  enjoys.  Imitation  of  this  character 
involves  a  species  of  inference,  and  its  occurrence  in  animals 
is  less  \\idespread  than  was  commonly  supposed  some 
years  ago.  Thorndike  tested  cats  and  dogs  with  the  view 
of  ascertaining  if  they  could  learn  to  get  out  of  a  puzzle 
box  by  seeing  other  cats  and  dogs  get  out.  After  witness- 
ing a  number  of  times  the  successful  exits  of  the  animals 
which  had  learned  the  means  of  escape  these  cats  and 
dogs  were  unable  to  get  out  by  themselves  any  more 
quickly  than  other  individuals  which  had  not  the  ad- 
vantage of  seeing  how  the  escape  was  effected.  In  all  the 
experiments  performed  the  animals  did  not  show  the  least 
tendency  to  imitate  the  performances  of  their  successful 
comrades.  The  experiments  of  Cole  and  Davis  on  the  rac- 
coon yielded  similar  negative  results. 

Small,  in  his  studies  on  the  white  rat,  finds  evidence  of  a 
very  simple  kind  of  imitation,  but  no  clear  cases  of  imitation 
of  the  inferential  type.  When  one  rat  sees  another  digging 
it  is  apt  to  dig  also,  and  when  one  runs  over  a  box  it  is  apt 
to  be  followed  by  others.  Berry,  who  has  made  a  more 
thorough  study  of  imitation  in  the  white  rat,  found  better 
evidence  of  intelligent  imitation.  "When  two  rats  were 
put  into  the  box  together,  one  rat  being  trained  to  get  out 
of  the  box  and  the  other  untrained,  at  first  they  were  in- 
different to  each  other's  presence,  but  as  the  untrained  rat 
observed  that  the  other  one  was  able  to  get  out  while  he 
was  not,  a  gradual  change  took  place.  The  untrained  rat 
began  to  watch  the  other  closely;  he  followed  him  all  about 
the  cage,  standing  up  on  his  hind  legs  beside  him  at  the  string, 
and  pulling  it  after  he  had  pulled  it,  etc.  We  also  saw  that 
when  he  was  put  back  the  immediate  vicinity  of  the  loop 
was  the  point  of  greatest  interest  for  him,  and  that  he  tried 


256  THE  INTELLIGENCE  OF  MAMMALS 

to  get  out  by  working  at  the  spot  where  he  had  seen  the 
trained  rat  try." 

It  is  a  significant  fact  that  the  untrained  rat  manifested 
little  interest  in  the  actions  of  the  trained  one  until  he  found 
the  latter  could  make  his  escape  from  the  cage.  If  a  new 
rat  were  put  in  the  cage  the  untrained  rat  would  follow  him 
also,  but  if  the  new  rat  did  not  get  out  his  companion 
would  soon  cease  to  follow  and  imitate  him.  It  is  impossible 
to  regard  such  imitation  as  this  as  a  series  of  congenital 
responses  to  the  perception  of  particular  movements.  The 
rat  imitates  with  the  aim  of  getting  out  of  the  cage  and 
apparently  recognizes  in  the  movements  of  his  companion 
the  means  of  deliverance.  Further  evidence  of  intelligent 
imitation  is  furnished  by  Berry^s  work  on  imitation  in 
cats. 

The  experiments  of  Hobhouse  yielded  many  indications 
of  imitation  of  the  inferential  type,  although  most  of  them 
leave  something  to  be  desired  in  the  way  of  conclusiveness. 
Hutchinson  in  his  valuable  book  on  Dog  Breaking  says 
that  dogs  may  be  taught  tricks  much  more  readily  if  they 
see  other  dogs  perform  the  tricks  and  obtain  a  reward 
for  it. 

A  general  consideration  of  the  literature  on  imitation  in 
animals  justifies  us,  I  think,  in  concluding  that  a  certain 
amount  of  intelligent  imitation  occurs  in  animals  below  the 
monkeys,  but  it  must  be  admitted  that  there  is  but  a 
small  amount  of  reliable  data  upon  which  to  base  an 
opinion. 

We  have  spoken  of  instinctive  and  intelligent  imitation 
as  if  they  constituted  two  discrete  classes  of  behavior.  It 
is  important,  I  believe,  in  studying  this  subject  to  recognize 
the  kinship  and  transitional  stages  between  these  two  kinds 
of  imitative  activity.    Animals  tend  to  imitate  those  acts 


THE  INTELLIGENCE  OF  MAMMALS  257 

which  are  most  closely  related  to  their  own  instinctive 
movements.  There  may  be  no  awareness  of  any  advantage  to 
be  gained  thereby,  as  in  the  f amihar  imitation  of  sounds  by 
birds.  Magpies,  ravens,  mocking-birds  and  especially  parrots 
attempt  to  repeat  various  sounds  which  they  are  accustomed 
to  hear.  Their  efforts,  imperfect  at  first,  improve  with 
repetition.  Making  a  sound  like  one  which  is  heard  seems  for 
some  reason  to  afford  these  birds  an  agreeable  experience. 
As  Baldwin  would  say,  hearing  the  sound  is  followed  by  an 
effort  to  "reinstate  the  stimulus"  and  so  gives  rise  to  a 
"circular  reaction."  Variations  of  its  own  notes  which 
approach  the  sounds  heard  become  "stamped  in,"  and  the  bird 
gradually  comes  to  imitate  the  sounds  more  closely,  much 
as  we  gradually  improve  upon  the  accuracy  of  our  own 
movements  in  playing  ball  or  tennis.  In  this  imitation  we 
need  suppose  no  element  of  transferred  association.  Based 
perhaps  upon  a  primary  instinct  to  utter  notes  in  response 
to  the  notes  which  it  hears,  which  we  often  find  in  young 
birds,  the  tendency  of  birds  to  imitate  sounds  involves  but 
the  rudimentary  form  of  intelligence  required  for  the  forma- 
tion of  simple  associations. 

There  is  comparatively  little  imitation  which  is  based  on 
a  cold  calculation  of  the  advantages  derived  from  copying 
another  animal.  The  imitation  which  is  shown  in  a  certain 
stage  of  the  life  of  the  child  is  intimately  related  to  a  certain 
satisfaction  derived  from  attaining  conformity  to  copy. 
This  trait  the  child  has  in  common  with  the  bird.  Probably 
our  own  unconscious  or  half  conscious  imitation  of  the  pro- 
nunciation and  mannerisms  of  the  people  we  live  with  is  a 
phenomenon  of  a  similar  kind.  How  far  this  kind  of 
imitation  occurs  in  the  mammals  has  not  been  clearly 
brought  out,  but  there  are  many  indications  of  its  influence 
in  several  of  our  common  domesticated  species. 

17 


258  THE  INTELLIGENCE  OF  MAMMALS 

BIBLIOGRAPHY 

Allen,  J.     The  Associative  Processes  of  the  Guinea  Pig.     Jour. 

Comp.  Neur.  Psych.,  14,  293,  '04. 
Ament,  W.     Ein  Fall  von  Ueberlegung   beim    Hund.     Arch.    ges. 

Psych.,  6,  249,  ^05. 
Berry,  C.  S.    The  Imitative  Tendencies  of  White  Rats.    Jour. 

Comp.  Neur.  Psych.,  16,  333,  '06. 

An  Experimental  Study  of  Imitation  in  Cats,  1.  c.  18,  1,  '08. 
BosTok,  F.     The  Training  of  Wild  Animals,  N.  Y.,  '03. 
Burroughs,  J.     Do  Animals  Think?     Harpers  Mon.,  110,  354,  '05. 
Carr,  H.  and  Watson,  J.  B.     Orientation  in  the  White  Rat.    Jour. 

Comp.  Neur.  Psych.  18,  27,  '08. 
Cesaresco,  E.  M.    The  Psychology  and  Training   of  the  Horse, 

London,  '06. 
Cole,   L.   W.    Concerning   the   Intelligence   of   Raccoons.    Jour. 

Comp.  Neur.  Psych.,  17,  211,  '07. 
Davis,  H.  B.    The  Raccoon:  A  Study  in  Animal  Intelligence.    Am. 

Jour.  Psych.,  18,  479,  '07. 
Freund,    F.    Der  "kluge"    Hans.    Ein    Beitrag  zur  Aufklarung. 

BerHn,  04. 
Hachet-Souplet,     W.     Examen    Psychologique     des     Animaux. 

Paris,  '00. 
Hobhouse,  L.  T.     Mind  in  Evolution.     London,  '01. 
Hutchinson,  W.  N.     Dog  Breaking,  6th.  ed.,     London,  '79. 
Mills,  T.  W.     The  Nature  and  Development  of  Animal  Intelligence, 

N.  Y.,  '98. 

The  Nature  of  Animal  Intelligence  and  the  Methods  of  Investi- 
gating It.    Psych.  Rev.,  6,  262,  '99. 
Pfungst,  O.     Das  Pferd  von  Herr  von  Osten  (Der  kluge  Hans), 

Leipzig,  '07.     Translation,  "  Clever  Hans, "  N.  Y.,  '11. 
Small,  W.  S.    An  Experimental  Study  of  the  Mental  Processes  of 

the  Rat.  Am.  Jour.  Psych.,  11,  133,  '99,  and  1.  c.  12,  206,  '00. 
Thompson-Seton,  E.  Life-Histories  of  Northern  Animals,  N.  Y.  '09. 
Thorndike,    E.    L.    Animal    Intelligence.    Psych.  Rev.  Monogr. 

Suppl.,     Vol.  2,  No.  4,  '98. 

Do  Animals  Reason?    Pop.  Sci.  Mon.,  55,  480,  '99. 

A  Reply  to  "  The  Nature  of  Animal  Intelligence  and  the  Methods 

of  Investigating  It."     Psych.  Rev.,  6,  412,  '99. 
ToENJiES,   H.    Principles    of   Animal    Understanding,  N.  Y.,  '06. 
Watson,  J.  B.     Animal  Education,  Chicago,  '03. 
Yerkes,  R.  M.    The  Dancing  Mouse,  N.  Y.,  '07. 


THE  INTELLIGENCE  OF  MAMMALS  259 

Yoakum,  C.  S.     Some  Experiments  on  the  Behavior  of  Squirrels. 

Jour.  Comp.  Neur.  Psych.,  19,  541,  '09. 
Zell,  T.    Das  rechnende  Pferd.     Ein  Gutachten  Gber  den  "  klugen 

Hans  "  auf  Gnind  eigener  Beobachtungen,  Berlin,  *04. 

1st  das  Tier  unvernunftig?    Neue  Einblick  in  die  Tierseele. 

Stattgart,  Fr.,  '04. 
ZuRN,  F.  A.    Die  intellektuellen  Eigenschaften  (Geist  und  Seele) 

des  Pferdes.    Stuttgart,  '99. 


CHAPTER  XIII 
THE  MENTAL  LIFE  OF  APES  AND  MONKEYS 

"L'Homme  ne  poss^de  aucune  aptitude  psychique  fondamentale 
qui,  k  un  moindre  degr^,  ne  se  manifeste  chez  certaines  B^tes." — 
Milne-Edwards,  Legons  sur  la  Physiologie,  T.  14. 

"If  no  organic  being  excepting  man  had  possessed  any  mental 
power,  or  if  his  powers  had  been  of  a  wholly  different  nature  from 
those  of  the  lower  animals,  then  we  should  never  have  been  able  to  con- 
vince ourselves  that  our  high  faculties  had  been  gradually  developed. 
But  it  can  be  shown  that  there  is  no  fundamental  difference  of  this 
kind.  We  must  also  admit  that  there  is  a  much  wider  interval  in 
mental  power  between  one  of  the  lowest  fishes,  as  a  lamprey  or 
lancelet,  and  one  of  the  higher  apes,  than  between  an  ape  and  man; 
yet  this  interval  is  filled  by  numberless  gradations." — Darwin 
Descent  of  Man. 

I  have  reserved  for  a  separate  chapter  a  consideration  of 
the  mental  powers  of  the  animals  most  closely  related  to 
ourselves.  Our  simian  cousins  have  long  enjoyed  the  reputa- 
tion of  attaining,  next  to  man,  the  highest  psychic  develop- 
ment of  any  members  of  the  animal  kingdom.  Much  has 
been  written  concerning  their  powers  and  performances, 
but  their  psychology  has  been  investigated  far  less  exten- 
sively and  thoroughly  than  the  importance  of  the  subject 
demands.  It  is  particularly  unfortunate  that  we  know  so 
little  of  the  most  anthropoid  of  the  ape  tribe.  Until  recently 
it  has  been  exceedingly  difficult  to  keep  the  larger  apes  long  in 
captivity,  and  psychologists  have  had  few  opportunities  to 
subject  them  to  systematic  experimentation.  Further  diffi- 
culties are  encountered  owing  to  the  large  size  and  strength 
of  these  animals,  especially  when  these  characteristics  are 
combined,  as  is  often  the  case,  with  an  unreliable  or  intract- 
able disposition. 

260 


MENTAL  LIFE  OF  APES  AND  MONKEYS       261 

It  is  probable  that  further  study  may  reveal  in  the  anthro- 
poid apes  a  higher  psychic  development  than  is  found  among 
the  smaller  members  of  the  monkey  tribe  from  which  has 
been  derived  most  of  our  knowledge  of  the  simian  mind. 
Our  account  therefore  will  have  to  do  mainly  with  the  smaller 
species  which  are  more  commonly  and  easily  kept  in  captivity 
and  which  are  more  distantly  related  to  the  human  family. 

Thorndike  has  performed  several  series  of  experiments 
with  three  Cebus  monkeys  by  placing  food  in  boxes  which 
could  be  opened  by  working  one  or  more  mechanical 
devices.  The  times  taken  by  the  monkeys  in  learning  how 
to  open  the  boxes  were  recorded,  and  it  was  found  that 
associations  were  formed  much  more  quickly  than  in  cats, 
dogs  and  chicks.  "Whereas  the  latter,"  says  Thorndike, 
"were  practically  unanimous  save  in  the  cases  of  the  very 
easiest  performances,  in  showing  a  process  of  gradual 
learning  by  a  gradual  elimination  of  unsuccessful  movements, 
and  a  gradual  reinforcement  of  the  successful  one,  these  are 
unanimous,  save  in  the  very  hardest,  in  showing  a  process  of 
sudden  acquisition  by  a  rapid,  often  apparently  instanta- 
neous, abandonment  of  the  unsuccessful  movements  and  a 
selection  of  the  appropriate  one  which  rivals  in  suddenness 
the  selections  made  by  human  beings  in  similar  performances." 
This  fact,  according  to  Thorndike,  does  not  show  that 
monkeys  reason  or  even  have  ideas.  Their  greater  clearness 
of  vision,  the  greater  number  and  precision  of  their  move- 
ments, and  their  greater  curiosity  are  factors  which,  aside 
from  the  superior  development  of  their  brains,  give  the 
monkeys  an  advantage  over  cats  and  dogs,  in  acquiring  new 
associations.  The  monkey  mind  shows  an  advance  over 
that  of  the  lower  mammals  in  the  greater  number,  deHcacy, 
complexity  and  permanence  of  its  associations,  and  the 
readiness  with  which  associations  are  formed,  but  in  their 


262       MENTAL  LIFE  OF  APES  AND  MONKEYS 

method  of  learning,  according  to  Thomdike, "  the  monkeys  do 
not  advance  far  beyond  the  generalized  mammalian  type." 

Like  his  cats  and  dogs,  Thordike's  monkeys  failed  to  learn 
acts  by  being  put  through  them.  They  also  failed  to 
leam  acts  which  they  had  seen  the  experimenter  perform  a 
large  number  of  times.  Neither  did  they  appear  to  imi- 
tate one  another.  If  one  monkey  had  learned  to  get  into 
a  puzzle  box,  other  monkeys  failed  to  leam  to  do  so  after 
witnessing  his  successful  performance.  The  traditional  belief 
in  the  propensity  of  monkeys  to  imitate,  therefore,  receives 
no  justification  at  Thorndike's  hands. 

The  poor  opinion  of  the  monkey  mind  to  which  Thomdike 
was  led  by  his  experiments  is  very  different  from  the  one 
commonly  held,  and  most  people  would  be  inclined  to  regard 
the  individuals  he  experimented  with  as  rather  sorry  speci- 
mens of  the  monkey  family.  Experiments  on  one  species 
form  an  inadequate  basis  upon  which  to  base  conclusions 
regarding  simian  intelligence  in  general,  and  it  is  not  sur- 
prising that  other  investigators  of  the  behavior  of  monkeys 
should  have  obtained  results  more  creditable  to  the  mental 
ability  of  these  animals.  Some  very  interesting  investiga- 
tions have  been  conducted  by  Hobhouse  upon  a  rhesus 
monkey  which  he  called  Jimmy,  and  a  chimpanzee  which 
for  reasons  of  his  own  he  named  the  Professor.  Jimmy  was  an 
active  creature  of  rather  irritable  temper  and  very  fond  of 
baked  potato,  which  proved  an  excellent  incentive  to  the 
overcoming  of  obstacles.  The  Professor,  on  the  other  hand, 
was  very  timid  and  unsociable,  and  could  be  managed  only 
with  difficulty. 

Both  of  these  monkeys  showed  a  certain  power  of  adapting 
means  to  ends  in  employing  one  object  in  order  to  get  another 
within  reach.  The  Professor  when  first  received  from  the 
zoological  gardens  had  already  acquired  the  habit  of  throw- 


MENTAL  LIFE  OF  APES  AND  MONKEYS       263 

ing  his  rug  over  objects  at  some  distance  from  his  cage  and 
pulling  them  in.  If  a  nut  or  piece  of  banana  was  placed  be- 
yond his  reach  outside  his  cage  he  would  get  his  rug  from  his 
bed,  stuff  it  with  considerable  labor  between  the  bars  of  his 
cage,  and  throw  it  like  a  net  over  the  desired  object.  He 
was  taught  to  substitute  a  stick  for  the  rug  and  succeeded 
in  employing  it  to  secure  bits  of  food.  The  next  day  he 
learned  to  use  a  short  stick  in  order  to  reach  a  longer  one 
with  which  he  could  secure  a  piece  of  banana.  "  I  put  my 
stick,*'  says  Hobhouse,  "out  of  his  reach,  and  a  piece  of 
banana  beyond  it  again,  while  I  gave  him  a  short  stick.  He 
did  not,  however,  use  it  until  I  first  pushed  the  big  stick  about 
with  it.  He  then  made  an  attempt  to  reach  my  stick  with 
the  short  one,  but  without  success.  I  then  gave  him 
rather  a  larger  stick,  with  which  he  at  once  tried  to  reach 
mine,  but  instead  of  getting  hold  of  it  he  knocked  it  slant- 
wise, so  that  one  end  w^as  farther  off  from  him  than  before, 
and  one  end  nearer.  He  now  directed  his  stick  to  the 
nearer  end,  pulled  mine  in,  and  with  its  aid  reached  the 
banana." 

The  chimpanzee  would  use  his  stick  in  different  ways 
according  to  circumstances.  "The  banana  was  generally 
given  him  inside  a  cigar  box.  He  would  reach  out  with 
his  stick  at  the  box,  and  sweep  it  round  by  a  radial  motion, 
so  that  in  so  doing  he  was  not  obeying  the  natural  impulse 
to  draw  it  straight  toward  him,  but  merely  was  bringing 
it  to  a  point  to  which  he  could  afterward  go  and  get  it.  One 
half  of  his  cage,  however,  was  covered  with  plate  glass, 
so  that  if,  in  describing  a  quarter  circle  he  swept  the  box  up 
against  the  glass  he  could  not  reach  it  at  once  with  his  arm. 
He  would  then  alter  the  motion,  and  rake  with  the  point  of  the 
stick,  drawing  the  box  in  in  a  straight  line.  "Wlien  he  had  to 
fish  for  a  box  close  to  the  wall,  he  would  take  trouble  to  get 


264       MENTAL  LIFE  OF  APES  AND  MONKEYS 

his  stick  in  between  the  wall  and  the  box,  showing  that  he 
was  quite  aware  of  the  way  in  which  he  had  to  push  it." 

Jimmy,  like  the  Professor,  learned  to  use  a  stick  with  which 
to  draw  in  objects,  and  he  also  learned  to  use  a  short  stick  in 
order  to  reach  a  larger  one  with  which  he  could  obtain 
his  food.  **If  stick  and  fruit  were  both  too  far  from  him, 
he  would  try  to  help  himself  by  pulling  the  whole  board  on 
which  they  lay  toward  him;  and  he  seemed  quite  clear  as  to 
the  necessity  of  getting  the  stick  beyond  the  thing  that  he 
was  pulling.  If  it  lay  just  on  his  side  of  the  apple  or  nut 
he  would  be  careful  not  to  knock  the  apple  away,  but  to 
get  the  stick  well  beyond  it." 

Jimmy^s  resourcefulness  in  attaining  his  ends  was  often 
quite  remarkable.  Given  a  child's  skipping  rope  with 
which  to  obtain  a  piece  of  bread  placed  outside  his  cage  he 
made  a  cast  at  it  with  the  wooden  handle  and  finally  drew 
it  within  reach.  After  this  he  would  use  a  cord  and  then  a 
wire  for  a  similar  purpose.  Says  Hobhouse,  '^I  put  a  piece 
of  onion  in  a  basket  within  reach  of  his  stick.  After  first  refus- 
ing any  effort,  he  tried  to  reach  it  with  the  stick,  and  failed. 
In  reaching  toward  it,  he  found  the  big  box  (one  of  Jack's 
boxes,  which  he  also  used)  lying  across  his  chain,  and  pre- 
venting his  reaching  forward.  He  threw  the  box  off.  Hav- 
ing failed  with  the  stick,  he  will  not  try  it  again,  but  makes 
wild  efforts  to  throw  the  rope.  Then  he  actually  rolls  the 
box  at  the  food;  then  goes  off  and  gets  the  dust  sheet  from 
the  chair  and  tries  unskilfully  to  sweep  at  it;  finally  makes  a 
longer  stretch,  and  just  reaches  it  with  his  own  claw." 

Jimmy  also  learned  to  use  a  chair  or  a  box  in  order  to  get 
objects  otherwise  out  of  reach.  An  onion  was  placed  on 
top  of  a  table  and  his  chain  was  adjusted  so  that  he  could 
reach  the  table,  but  not  the  onion.  After  he  was  allowed  to 
reach  the  latter  by  using  a  box  which  was  placed  so  that  he 


MENTAL  LIFE  OF  APES  AND  MONKEYS       265 

could  climb  upon  it  the  box  was  removed.  At  first  Jimmy 
did  not  try  to  move  the  box  back,  but  began  to  explore  it  and 
opened  the  lid  upon  which  he  climbed  and  reached  the  food. 
The  next  time  he  tried  to  pull  the  table  toward  him. 
After  a  number  of  different  manoeuvers  to  get  the  food  he 
learned  to  drag  the  box  or  a  chair  toward  the  table  and 
by  this  means  reach  the  object  he  was  seeking. 

In  another  experiment  a  box  was  placed  between  the 
fastening  of  his  chain  and  a  piece  of  potato  which  was  just 
out  of  reach,  but  which  could  be  reached  if  the  chain  did 


Fig.  16. — "Jimmy"  using  a  box  in  order  to  reach  food  on  the  table. 
(After  Hobhouse). 

not  pass  around  the  box.  Jimmy  soon  came  back  to  the 
box,  moved  it  out  of  the  way  and  got  his  potato.  He  re- 
peated the  performance  two  more  times  without  hesitation. 
When  his  chain  was  shortened  by  a  heavy  fender  being 
placed  over  it  he  would  come  back  and  lift  off  the  obstacle. 
He  recognized,  apparently,  that  the  box  and  the  fender  pre- 
vented him  from  going  as  far  as  he  otherwise  might. 

Similar  adjustment  of  means  to  ends  was  shown  by  the 
monkey  kept  by  Miss  Romanes,  which  she  describes  as 
follows:  "His  chain  is  fastened  to  the  marble  slab  of  a 
washhandstand,  placed  on  the  floor  against  the  wall.  It  is 
too  heavy  for  him  to  pull  along  by  his  chain  without  hurting 
himself,  so  when  he  desires  to  do  any  mischief  which  is 


266       MENTAL  LIFE  OF  APES  AND  MONKEYS 

beyond  the  reach  of  his  chain  he  dehberately  goes  to  the 
marble  and  pushes  an  arm  down  between  the  upright  part 
of  it  and  the  wall,  until  he  has  moved  the  whole  slab 
sufficiently  far  from  the  wall  to  admit  of  his  slipping  down 
behind  the  upright  part  himself.  He  then  places  his  back 
against  the  wall  and  his  four  hands  against  the  upright 
part  of  the  marble,  and  pushes  the  slab  as  far  as  he  can 
stretch  his  long  legs.  He  only  does  this,  however,  when  he  is 
bent  on  mischief,  as  the  fact  of  food  being  beyond  the  reach 
of  his  chain  does  not  furnish  a  strong  enough  inducement  to 
lead  him  to  take  so  much  exertion." 

Another  interesting  incident  in  this  connection  is  furnished 
by  Prof.  Mobius.  A  chimpanzee,  Molli,  was  confined  in  a 
wooden  cell.  When  someone  from  the  outside  was  driving 
a  nail  into  the  wall  of  her  cell,  and  she  saw  the  nail  coming 
through  the  board,  she  went  to  her  drinking  vessel,  took  it 
in  both  hands  and  pounded  the  nail  back  again. 

The  use  of  implements  for  various  purposes  is,  according 
to  the  testimony  of  several  writers,  a  not  uncommon  feature 
of  monkey  behavior.  Prof.  Cope,  in  describing  the  perform- 
ances of  a  Cebus  capucinus  says:  "He  then  used  the  strap 
in  a  novel  way.  He  was  accustomed  to  catch  his  food  with 
his  hands  when  thrown  to  him.  Sometimes  it  fell  short 
three  or  four  feet.  One  day  he  seized  his  strap  and  took 
pretty  correct  aim  and  finally  drew  the  pieces  to  within 
reach  of  his  hand.  This  performance  he  constantly  repeats, 
hooking  and  pulling  the  articles  to  him  in  turns  of  the  strap. 
Sometimes  he  loses  hold  of  the  strap.  If  the  poker  is  handed 
him  he  uses  it  with  some  skill  for  the  recovery  of  the  strap. 
After  punishment  the  animal  would  only  exert  himself  in 
this  way  when  not  watched;  as  soon  as  an  eye  was  directed 
to  him  he  would  cease.'' 

Witmer  relates  that  Peter,  a  trained  chimpanzee  which 


MENTAL  LIFE  OF  APES  AND  MONKEYS       267 

he  had  for  a  time  under  observation,  would  use  a  hammer  to 
drive  nails  with,  string  beads  on  a  cord,  select  the  proper 
key  from  a  bunch  and  unlock  a  padlock,  and  strike  a  match, 
light  a  cigarette,  and  smoke  it  with  an  expression  of  serene 
contentment.  Peter  developed  a  remarkable  expertness 
in  riding  a  bicycle,  a  feat  which  he  was  accustomed  to  per- 
form during  his  public  exhibitions. 

Mr.  Belt  tells  of  a  Cebus  he  observed  which  would  use  a 
stick  with  which  to  draw  objects  toward  him.  One  day 
some  bird  skins  were  placed,  as  was  thought,  far  beyond  his 
reach,  but  he  took  his  swing  and  used  it  so  as  to  bring  the 
skins  near  enough  to  be  seized.  He  also  secured  some 
jelly,  which  was  set  out  to  cool,  in  the  same  way. 

Miss  Romanes  in  her  account  of  the  behavior  of  a  Cebus, 
from  which  we  have  already  quoted,  states  that  "if  a  nut 
or  any  object  he  wishes  to  get  hold  of  is  beyond  the  reach  of 
his  chain,  he  puts  out  a  stick  to  draw  it  toward  him,  or  if  that 
does  not  succeed,  he  stands  upright  and  throws  a  shawl  back 
over  his  head,  holding  it  by  the  two  comers  so  that  it  faUs 
down  his  back;  he  throws  it  forward  with  all  his  strength,  still 
holding  on  by  the  corners;  thus  it  goes  out  far  in  front  of  him 
and  covers  the  nut,  which  he  then  draws  toward  him  by 
pulling  in  the  shawl."  The  same  monkey  when  given  a 
hammer  to  crack  a  nut  with  used  it  effectively  for  that 
purpose.  We  have  the  testimony  of  Cuvier  to  the  effect 
that  an  orang  would  pull  a  chair  from  one  end  of  the  room 
to  the  other  so  that  by  standing  upon  it  he  could  open  a 
latch,  but  how  the  act  was  originally  learned  we  are  not 
informed. 

The  use  of  sticks  and  stones  by  apes  and  monkeys  has 
been  described  by  several  writers.  Pechuel-Losche,  who  has 
had  excellent  opportunities  for  observing  baboons  in  their 
native  habitat,  has  come  to  the  conclusion  that  the  accounts 


268       MENTAL  LIFE  OF  APES  AND  MONKEYS 

of  these  animals  throwing  stones  at  their  pursuers  rest  upon 
inaccurate  observation;  the  stones,  he  thinks,  were  merely 
knocked  down  unintentionally  during  the  flight  of  these 
animals  up  a  declivity.  Stories  of  apes  throwing  down 
fruit  and  other  objects  from  trees  he  thinks  are  based 
on  the  fact  that  these  things  are  simply  dropped  when  the 
animals  are  put  to  flight.  Father  Wasmann,  who  eagerly 
adopts  these  conclusions,  proceeds  to  admonish  us  that 
the  boasted  intelligence  of  apes  is  entirely  illusory.  ''Had 
apes  themselves,"  he  tells  us  ''but  a  trace  of  intelligence, 
they  would  have  invented,  long  ago  even  in  their  free  state 
of  nature  the  use  of  a  few  simple  means  of  defence,  such 
as  branches  and  stones.  But  why  did  they  not?  The  only 
possible  answer  is:  because  they  evidently  have  no  intelli- 
gence. Not  the  brain  alone  makes  man  an  intelligent  being, 
but  his  spiritual  soul,  and  this  spiritual  soul  is  wanting  in 
the  highest  apes  as  well  as  in  the  insects. '^ 

If  the  failure  of  apes  to  use  weapons  of  defence  is  indicative 
of  lack  of  intelligence,  we  may  fairly  conclude  that  where 
weapons  are  used  there  is  evidence  that  intelligence  exists. 
Whether  or  not  baboons  use  sticks  and  stones  in  the  way 
alleged,  there  is  good  evidence  that  other  members  of  the 
ape  tribe  sometimes  employ  them  as  a  means  of  attack. 
Miss  Romanes  says  regarding  her  Cebus :  "  To-day  a  strange 
person  (a  dressmaker)  came  into  the  room  where  he  is  tied 
up,  and  I  gave  him  a  walnut  that  she  might  see  him  break 
it  with  his  hammer.  The  nut  was  a  bad  one,  and  the  woman 
laughed  at  his  disappointed  face.  He  then  became  very 
angry,  and  threw  at  her  everything  he  could  lay  hands 
on;  first  the  nut,  then  the  hammer,  then  a  coffee-pot  which 
he  seized  out  of  the  grate,  and,  lastly  all  his  own  shawls. 
He  throws  things  with  great  force  and  precision  by  holding 
them  in  both  hands,  and  extending  his  long  arms  well  back 


MENTAL  LIFE  OF  APES  AND  MONKEYS       269 

over  his  head  before  projecting  the  missile,  standing  erect 
ihe  while."  At  a  later  period  Miss  Romanes  says  of  him: 
"  When  he  throws  things  at  people  now  he  first  runs  up  the 
bars  of  the  clothes-horse;  he  seems  to  have  found  out  that 
people  do  not  care  much  for  having  things  thrown  at  their 
feet,  and  he  is  not  strong  enough  to  throw  such  heavy  objects 
as  a  poker  or  a  hammer  at  peoples'  heads;  he  therefore 
mounts  to  a  level  with  his  enemy's  head,  and  thus  succeeds 
in  sending  his  missile  to  a  greater  height  and  also  to  a  greater 
distance."  Darwin  states  in  his  "Descent  of  Man:"  "As 
I  have  repeatedly  seen,  a  chimpanzee  will  throw  any  object 
at  hand  at  a  person  who  offends  him."  And  other  well 
attested  observations  could  be  quoted  to  the  same  effect. 
Despite  the  contention  of  Wasmann  we  must  admit,  I 
think,  that  the  use  of  objects  by  apes  as  weapons  of  attack 
rests  upon  a  fair  basis  of  testimony. 

A  certain  degree  of  foresight  seemed  to  be  manifested 
by  Miss  Romanes'  monkey  in  the  way  in  which  he  disen- 
tangled his  chain  when  it  became  wound  around  a  clothes 
horse  which  was  given  him  to  run  upon.  "He  looks  at  it 
intently  and  pulls  it  with  his  fingers  this  way  and  that,  and 
when  he  sees  how  the  turns  are  taken,  he  deliberately  goes 
round  and  round  the  bars,  first  this  way,  then  that,  until 
the  chain  is  quite  disentangled.  He  often  carries  his 
chain  grasped  in  his  tail  and  held  high  over  his  back  to  keep 
it  from  getting  in  the  way  of  his  feet." 

Mr.  Hobhouse's  Jimmy  was  not  so  circumspect.  "He 
would,  however,  as  a  rule,  undo  a  single  twist  by  retracing 
his  steps,  and  sometimes  would  undo  a  more  complicated  one 
by  a  developed  form  of  the  method  of  trial  and  error,  which 
consisted  in  this:  that  each  time  he  felt  the  cord  shortening 
on  him,  he  would  go  back  the  way  he  had  come."  This 
method  was  usually  effective,   but  Jimmy  showed  little 


270       MENTAL  LIFE  OF  APES  AND  MONKEYS 

improvement  in  its  application.  He  made  but  a  moderate 
success  in  disentangling  his  chain  when  it  became  twisted 
around  among  the  legs  of  a  chair  or  table. 

While  monkeys  are  generally  credited  with  unusual  powers 
of  imitation,  the  experiments  of  recent  years  have  shown 
that  imitation  is  far  less  frequent  than  was  supposed.  Thorn- 
dike  tried  to  find  if  monkeys  would  learn  to  enter  a  puzzle 
box  any  more  quickly  after  having  witnessed  a  number  of 
times  how  he  opened  the  various  fastenings.  Several 
kinds  of  boxes  were  used,  but  the  monkeys  did  not,  in  any 
case,  make  sufficient  progress  to  justify  the  conclusion  that 
they  learned  by  imitation.  Neither  did  monkeys  which 
failed  to  learn  how  to  enter  the  puzzle  boxes  after  several 
trials  imitate  others  which  had  learned  to  operate  the  fasten- 
ings. No  evidence  of  imitation  was  furnished  by  the  general 
behavior  of  these  animals,  but  since  two  of  the  monkeys 
were  on  very  unfriendly  terms  and  the  third  was  exceedingly 
timid  their  social  relations  were  not  such  as  to  favor  the 
imitation  of  one  another's  acts. 

Further  experiments  were  carried  on  by  Watson  on  four 
monkeys,  a  baboon,  a  Cebus,  and  two  rhesus  monkeys. 
Watson  tested  his  animals  by  performing  in  their  presence 
a  number  of  acts  which  resulted  in  securing  food,  such  as 
drawing  in  food  with  a  rake  or  a  cloth,  getting  it  from  a 
bottle  with  a  fork,  and  poking  it  out  of  a  glass  cylinder  with 
a  stick.  After  witnessing  his  operations  a  number  of  times 
there  was  no  effort  on  the  part  of  any  of  the  monkeys  to  get 
the  food  in  the  way  they  had  every  opportunity  to  see  was 
effective.  Experiments  with  puzzle  boxes  in  which  the 
monkeys  were  given  a  chance  to  imitate  either  the  experi- 
menter or  a  monkey  w^hich  had  already  learned  the  trick 
gave  the  same  negative  results. 

While  they  yielded  no  evidence  of  imitation  in  its  higher 


MENTAL  LIFE  OF  AFES  AND  MONKEYS         271 


Fig.  17. — Peter  at  dinner.     (By  permission  of  the  Psychological  Clinic.) 


272       MENTAL  LIFE  OF  APES  AND  MONKEYS 

forms  Watson's  monkeys  occasionally  performed  acts  which 
were  ''  suggestive  of  a  low  order  of  imitation/'  One  of  them 
found  a  hole  in  a  window  frame.  '*This  one  would  'peek' 
and  then  another  one  would  push  him  aside  and  'peek '  in  turn. 
This  was  observed  several  times."  Hirschlaff  in  his  account 
of  the  chimpanzee  Consul  describes  a  number  of  cases  in 
which  he  considers  that  imitation  was  unmistakably  shown, 
but  his  observations  are  not  described  in  sufficient  detail  to 
enable  one  to  judge  of  the  correctness  of  the  interpretation. 
Kinnaman  reports  a  few  cases  of  imitation  in  two  rhesus 
monkeys  which  he  studied.  In  one  instance  in  which  the 
female  pulled  out  a  olug  immediately  after  seeing  a  male  do 
it  the  evidence  for  imitation  is  not  entirely  conclusive,  since 
the  female  had  pulled  out  the  plug  on  previous  occasions. 
The  second  case  is  a  more  satisfactory  one  and  is  described 
as  follows:  "  Recalling  that  she  had  failed  to  work  the  bear- 
down  lever  for  opening  the  box.  ...  I  placed  it  before 
her.  She  rushed  up,  but  missing  the  plug  she  sat  dow^n. 
The  male  passed  her,  pushed  the  lever  down  and  procured 
the  food.  When  the  box  w^as  set  again  she  worked  the  lever 
and  took  the  food  in  the  same  way  that  he  had  done.  She 
manipulated  this  apparatus  several  times  immediately 
and  250  times  later  as  a  part  of  a  combination  lock.  Besides 
these,  once  when  the  male  peeped  under  the  bottom  of  one 
of  the  trees  the  female  came  and  peeped  in  the  same  manner." 

Mr.  Witmer  states  that  the  chimpanzee,  Peter,  twice  copied 
a  W  which  was  written  on  a  blackboard,  and  learned  to  repeat 
the  word  ''mama."  It  is  unfortunate  that  more  extended 
and  thorough  experiments  were  not  carried  out  with  so  prom- 
ising a  subject. 

The  experiments  of  Shepherd  on  imitation  in  rhesus 
monkeys  yielded  for  the  most  part  negative  results.  In  a 
few  cases,  however,  the  monkeys  showed  evidence  of  learn- 


MENTAL  LIFE  OF  APES  AND  MONKEYS       273 

ing  by  imitating  the  experimenter's  movements.  In  one 
such  case  a  piece  of  banana  was  suspended  by  a  string  a 
Uttle  beyond  the  monkey's  reach,  and  a  pole  was  so  ar- 
ranged that  when  it  was  swung  under  the  banana  the 
monkey  could  get  upon  it  and  reach  the  fruit.  A  monkey 
which  had  proven  himself  incapable  of  solving  the  problem 


Fig.  18. — Peter's  efforts  at  copying  letters  on  a  blackboard,  a.  Two 
superimposed  letters  drawn  by  Mr.  Witmer;  a.\  Peter's  copy  after  the  sec- 
ond tracing :  a-,  Peter's  second  effort  when  told  to  make  a  W  again.  (By 
permisson  of  the  Psychological  Clinic.) 

alone  was  shown  how  to  manipulate  the  pole.  After  wit- 
nessing the  experimenter  push  the  pole  under  the  banana 
the  monkey  gradually  learned  to  perform  the  act,  although 
it  was  some  time  before  he  manipulated  the  apparatus  with 
accuracy  and  dispatch. 

The  most  extensive  study  of  imitation  in  monkeys  has 
been  made  by  Mr.  Haggerty.     While  many  of  the  experi- 

18 


274       MENTAL  LIFE  OF  APES  AND  MONKEYS 

ments  gave  negative  results  and  many  others  leave  us  in 
doubt  as  to  the  interpretation  of  the  behavior  described, 
there  are  some  in  which  the  evidence  points  strongly  to- 
ward imitation.  The  monkeys  were  put  in  cages  in  which 
there  were  mechanical  devices  by  operating  which  they 
could  procure  food.  Each  monkey  was  given  five  trials  of 
fifteen  minutes  each  on  successive  days.  If  he  did  not 
secure  food  by  his  unaided  efforts  he  was  allowed  to  see 
another  monkey  operate  the  device  and  was  then  allowed 
to  try  again.  The  lessons  were  kept  up  until  100  tests  were 
made  before  the  monkey  was  dismissed  as  a  hopeless  failure. 
In  many  cases  monkeys  which  failed  to  operate  the  devices 
alone  did  so  after  watching  other  monkeys  work  them  one  or 
more  times.  The  attention  of  the  monkeys  was  usually 
stimulated  when  they  saw  other  monkeys  obtain  food  by 
working  the  devices.  Very  frequently  the  imitation  was 
not  perfect  at  first,  but  the  various  features  of  the  trick 
were  learned  one  after  the  other. 

There  is  a  possible  doubt  relative  to  the  interpretation  of 
Mr.  Haggerty's  experiments.  Since  the  monkey  which 
served  as  a  model  learned  the  trick  to  be  copied  it  is  possible 
that  the  monkey  which  performed  the  trick  after  watching 
him  may  have  learned  it  at  first  hand  also.  The  case  for 
imitation  can  be  made  out  only  by  the  accumulation  of  a 
sufficient  number  of  instances  to  rule  out  coincidences  and 
accidents.  How  far  Mr.  Haggerty  has  made  out  his  case 
can  be  judged  only  by  a  careful  study  of  the  details  given 
in  his  paper. 

Whether  apes  and  monkeys  reason  is  a  question  whose 
answer  depends  on  the  sense  in  which  the  term  reason  is 
employed.  If  we  define  reason  as  the  derivation  of  conclu- 
sions through  the  comparison  of  concepts  it  is  not  improbable 
that  no  animal  below  man  employs  this  faculty.     But  this 


MENTAL  LIFE  OF  APES  AND  MONKEYS       275 

is  far  from  implying  that  animals  cannot  perform  mental 
operations  which  are  essentially  inferential  in  their  nature. 
Reason,  as  has  been  stated  before,  is  not  a  faculty  which 
stands  sharply  marked  off  from  other  forms  of  mental 
activity.  Between  simple  perception  on  the  one  hand  and 
abstract  ratiocination  on  the  other  there  is  a  fundamental 
kinship  and  the  latter  process  may  be  connected  with  the 
first  by  numerous  intermediate  stages.  Monkeys,  in  all 
probability,  have  the  power  of  using  ideas  derived  from  their 
experience  as  a  means  of  reaching  practical  results.  The 
monkey  which  pulls  a  chair  or  stool  into  a  certain  position, 
gets  on  it  and  secures  food,  manifests  a  certain  power  of 
inference.  He  may  not  explicitly  reason:  "This  fruit  is 
beyond  my  reach;  if  I  had  something  to  get  upon  I  could 
secure  the  fruit;  this  chair  would  serve  my  purpose  and 
moreover  is  movable;  ergo,  I  will  pull  it  up  and  get  on  it."  In 
all  probability  a  man  in  a  similar  situation  would  not  reason 
as  explicitly  either.  He  would  perceive  that  the  object  was 
out  of  his  reach;  seeing  a  chair  the  idea  would  come  of  pulling 
it  up  and  getting  on  it,  and  the  idea  would  forthwith  issue 
in  the  proper  act.  There  would  be  no  formal  syllogism  gone 
through  with.  The  process  would  ordinarily  take  place 
so  quickly  that  he  would  scarcely  be  conscious  of  the  steps. 
It  is  true  that  the  man  might  think  about  the  matter  in  a 
very  complex  way,  and  employ  a  lot  of  abstract  and  general 
ideas,  but  this  process  would  be  dispensed  with  under  or- 
dinary conditions,  especially  if  he  were  in  a  hurry.  The 
man's  mental  operations,  even  in  their  simplest  form,  would 
nevertheless  be  in  the  nature  of  an  inference,  and  so  far  as 
we  can  judge  from  appearances,  the  same  statement  is  true 
of  the  mind  of  the  monkey.  The  latter  cannot,  in  all  proba- 
bility, think  the  situation  over  as  the  man  can  in  terms  of  a 
formal  syllogism,  but  he  has  the  more  immediately  useful 


276       MENTAL  LIFE  OF  APES  AND  MONKEYS 

faculty  of  arriving  at  the  practical  conclusion  as  to  the  kind 
of  action  which  the  conditions  call  for. 

Inferences  of  this  simple  practical  character  are  of  enor- 
mous value  to  such  creatures  as  monkeys.  They  suflBce 
also  for  the  greater  part  of  our  own  conduct,  at  least  in 
certain  walks  of  life.  Within  the  sphere  of  rational  pro- 
cedure in  ourselves  there  are  obviously  vast  differences  in 
the  complexity  and  abstractness  of  our  mental  operations, 
and  it  might  be  said  that  the  gap  between  the  animal  and  the 
human  mind  corresponds  roughly  with  the  difference  be- 
tween our  more  simple  and  our  more  complex  and  abstract 
thinking.  A  similar  gap  is  bridged  over  during  the  life 
of  every  individual  in  passing  from  infancy  to  maturity, 
and  we  may  well  conceive  the  mind  of  man  to  have  arisen 
from  the  animal  mind  by  a  similar  process  of  continuous 
development. 

Father  Wasmann  is  one  of  the  few  comparative  psycholo- 
gists of  note  who  hold  the  human  and  the  animal  mind  to  be 
fundamentally  distinct.  While  the  real  basis  of  his  opinion 
may  be  his  adherence  to  the  traditional  theology  which  he 
represents,  with  its  peculiar  views  as  to  man's  relation 
to  nature,  Wasmann  has  attempted  to  show  that 
animals  have  no  real  intelligence,  no  power  of  abstraction,  and 
no  power  of  rational  thought.  Even  should  we  grant  all 
this,  we  should  not  be  compelled  to  call  into  play  a  miracu- 
lous intervention  to  account  for  the  distinctive  attributes  of 
human  thinking.  Giving  names  to  particular  mental 
faculties  tends  to  exaggerate  their  distinctiveness,  as  was 
done  in  the  faculty  psychology  of  former  days.  When  we 
say  of  a  stage  of  mental  evolution,  here  there  is  reason  while 
at  a  stage  just  preceding,  reason  does  not  occur,  our  statement 
does  not  necessarily  imply  an  abrupt  break  or  sudden  step 
in  the  evolutionary  series.     As  Hobhouse  has  well  remarked: 


MENTAL  LIFE  OF  APES  AND  MONKEYS       277 

"  If  we  allow  reason  to  the  human  species  in  general,  and  yet 
restrict  it  to  that  species,  it  must  be  by  identifying  the  term 
reason  arbitrarily  with  a  certain  grade  in  the  development 
of  analysis.  It  would  be  true  to  say  that  abstract  or  ex- 
plicitly general  reasoning  emerges  in  the  level  of  intelligence 
under  consideration,  but  we  have  seen  that  abstractness 
is  only  one  side  of  generality,  and  that  the  generality  of 
human  as  opposed  to  animal  reasoning  is  once  more  primarily 
a  matter  of  explicitness.  At  bottom  the  function  of  mind  j 
in  this  as  in  the  lower  stages  is  to  organize  life  by  the  correla-  [ 
tion  of  experiences.  As  in  every  stage  of  mental  growth 
what  is  new  is  that  the  work  of  the  mind  becomes  on  the 
one  hand  more  explicit  or  articulate,  on  the  other,  more 
comprehensive  in  scope.'* 

Wasmann's  position  is  typical  of  the  attitude  which 
formerly  characterized  to  a  greater  extent  than  happily  is 
the  case  now  the  relation  of  theology  to  science.  It  is  the  aim 
of  science  to  explain  phenomena  in  terms  of  natural  laws. 
Theology,  on  the  other  hand,  has  always  been  interested  in 
finding  things  which  cannot  be  so  explained,  and  in  thus 
compelling  us  to  take  refuge  in  some  sort  of  supernatural 
intervention.  Gaps,  barriers,  discontinuities,  mysterious 
and  inexplicable  phenomena  have  always  afforded  her, 
therefore,  a  peculiar  satisfaction;  but  one  by  one  they  have 
been  obliterated  or  resolved,  and  there  can  be  little  doubt,  I 
believe,  that  the  question  of  the  continuous  evolution  of  the 
human  mind  will  go  the  way  of  the  others. 

While  there  is  little  to  indicate  that  the  apes  are  able  to 
reason  in  an  abstract  way,  it  is  obviously  absiu*d  to  attempt 
to  account  for  such  behavior  as  we  have  described  on  the 
basis  of  blind  sensori-motor  association.  It  is  not  going  too 
far,  I  think,  to  say  that  there  is  good  evidence  in  the  apes  and 
monkeys  for  the  existence  of  ideas  and  for  a  certain  power 


278       MENTAL  LIFE  OF  APES  AND  MONKEYS 

of  grasping  relations.  A  monkey  which  uses  a  stool  to 
enable  him  to  reach  an  object,  and  which  removes  an  obstacle 
from  his  chain  so  that  he  may  get  a  piece  of  food  can  scarcely 
be  said  to  be  devoid  of  a  certain  power  of  representation. 
Many  features  of  the  behavior  of  apes  and  monkeys  mark 
these  animals  as  belonging  to  a  distinctly  higher  psychic 
level  than  cats  and  dogs.  It  is  not  likely  that  a  cat  or  a  dog, 
with  all  due  allowance  for  its  physical  disabilities,  would 
employ  a  tool  with  which  to  pull  in  a  bit  of  food,  much  less 
use  a  stick  in  order  to  get  the  tool  for  this  purpose.  The 
monkey  looks  on  the  tool  as  a  means  to  an  end,  and  accord- 
ingly goes  after  it.  There  is  less  evidence  that  objects  have 
such  a  meaning  to  cats  and  dogs.  These  animals  might  try 
in  various  ways  to  reach  food  lying  on  a  table;  if  a  chau-  or 
box  were  at  hand  they  would  doubtless  mount  upon  it  in 
order  to  get  the  food.  But  even  after  seeing  chairs  and 
boxes  pulled  around  by  human  beings  any  number  of  times 
it  probably  would  not  occur  to  one  of  these  creatures  to  pull 
the  chair  or  box  into  position  for  its  own  use. 

The  mind  of  the  lower  mammals  is  pretty  closely  chained  to 
its  various  objects  of  perception.  It  may  have  ideas,  but 
they  are  lacking  in  "articulateness.^'  But  the  monkey 
seems  to  be  gifted  with  a  certain  degree  of  initiative;  things 
occur  to  him,  and  he  apparently  thinks  about  things  in  the 
effort  to  attain  a  particular  result.  He  shows  a  decided 
approach  to  ourselves  in  many  little  ways  of  doing  things. 
He  is  not  interested  merely  in  the  gratification  of  his  appe- 
tites; he  is  actuated  by  a  sort  of  intellectual  curiosity  in  re- 
gard to  objects.  Miss  Romanes'  monkey  would  almost  always 
set  an  orange  to  spinning  before  eating  it  just  for  the  fun 
of  seeing  it  go;  he  took  great  delight  in  breaking  objects  to 
pieces  and  in  overturning  things;  he  removed  a  bell  handle 
from  the  mantel  piece  which  involved   unscrewing   three 


MENTAL  LIFE  OF  APES  AND  MONKEYS       279 

screws;  and  when  he  was  chained  in  a  new  place  he  investi- 
gated for  hours  the  new  fastenings  of  his  chain.  The 
development  of  this  intellectual  interest — if  we  may  call  it 
such — in  regard  to  objects  affords  an  opportunity  for  further 
improvement;  it  leads  not  only  to  the  acquisition  of  new 
knowledge,  but  to  training  in  discrimination  and  practical 
judgment. 

BIBLIOGRAPHY 

Brehm,  a.  E.     Thierleben,  76. 

Cope,  E.  D.     Note  on  Intelligence  in  Monkeys.  Am.  Jour.  Sci.  and 

Arts  (3),  4,  147,  72;  also  in  Ann.  Mag.  Nat.  Hist.  (4),  10,  229, 

72. 
CuviER,  F.     Description  d'un  Orang-Outang,  et  Observations  sur 

ses  Facult^s  Intellectuelles.     Ann.  du  Mus.  Hist.  Nat.,  16,  1808. 
Darwin,  C.  R.     Descent  of  Man,  N.  Y.,  74. 
Garner,  R.  L.    The  Speech  of  Monkeys,  N,  Y.,  '92. 

Apes  and   Monkeys,   their  Life  and  Language,    Boston,   '00. 
Haggerty,    M.    E.     Imitation  in  Monkeys.   Journ.  Comp.  Neur. 

Psych.,  19,  337,  '09.     Imitation  in  Monkeys.     Century   Mag. 

'09.  544. 
Hartmann,  R.     Anthropoid  Apes,  N.  Y.,  '85. 
HiRscHLAFF,  L.    DcF  Schimpausc  Konsul;  ein  Beitrag  zur  ver- 

gleichenden  Psychologie.     Zeit.  f.  pad.  Psychol.,  7,  1,  '05. 
HoBHOUSE,  L.  T.     Mind  in  Evolution,  London,  '01. 
KiNNAMAN,  A.  J.     Mental  Life  of  two  Macacus  Rhesus  Monkeys  in 

Captivity.     Am.  Jour.  Psych.,  13,  98  and  173,  '02. 
MoBius,  K.     Zur  Psychologie  des  Schimpanse.     Zool.  Garten,  8, 

279,  '67. 
Romanes,  G.  J.    Animal  Intelligence,  N.  Y.,  '83. 

On  the  Mental  Faculties  of  the  Bald  Chimpanzee,  Anthropithetms 

calvus.     Proc.  Zool.  Soc,  London,  '89,  316. 
Shepherd,  W.     Some  Mental  Processes  of  the  Rhesus  Monkey. 

Psychol.  Rev.  Monogr.  Suppl.  12,  No.  52,  '10. 
SoKOLOwsKY,  A.   Beobachtungen  iiber  die  Psyche  der  Menschenaffen. 

Frankfurt,  '08. 
Thorndike,  E.  L.     The  Mental  Life  of  the  Monkeys,  Psychol.  Rev. 

Monogr.  Suppl  3,  No.  15,  '01. 

The  Intelligence  of  Monkeys.     Pop.  Sci.  Mon.,  54,  '01. 


280       MENTAL  LIFE  OF  APES  AND  MONKEYS 

Wallace,  A.  R.    The  Malay  Archipelago,  3d.  ed.,  London,  72. 
Watson,  J.  B.     Imitation  in  Monkeys.    Psych.  Bull.  5,  169,  '08. 

Some   Experiments   Bearing   upon   Color  Vision  in  Monkeys. 

Jour.  Comp.  Neur.  Psych.,  19,  1,  '09. 
WiTMER,  L.     A  Monkey  with  a  Mind.    Psych,  Clinic,  3,  179,  '09. 

Intelligent  Imitation  and  Curiosity  in  a  Monkey,  1.  c.  3, 225,  '10. 


INDEX 


INDEX 


Abbott,  C.  C,  intelligence  of  frogs, 

228 
Actinians,  effect  of  hunger  on,  150, 
151 
geotaxis,  222 

tidal  and  daily  rhythms,  157 
varied  reactions,  144-147, 159- 
161 
Adams,  G.  P.,  phototaxis  of  earth- 
worm, 47 
Adamsia,  food  taking,  151 
Adlerz,  G.,  imitation  in  ants,  210 

intelligence  of  wasp,  192 
^thalium,  rheotaxis,  37 
Aikins,  H.  A.  See  Hodge,  89 
Aiptasia,  varied  reactions,  146 
effect  of  hunger  on,  151 
habit  formation,  153 
Allabach,  L.  F.,  reactions  of  Metri- 

dium,  160 
Allen,  J.  ,258 

Alligator,  intelligence,  230 
Allolobophora,  positive  phototaxis 

in  weak  light,  47 
Ament,  W.,  258 
Ammophila,  homing,  194 

stinging  prey,  119,  120,  131, 

132 
using  stone  as  a  tool,  203,  204 
variability  of  instinct,  131, 132 
Amoeba,  adaptiveness  of  behavior, 
70,  71 
chemotaxis,  23,  26,  70 
electrotaxis,  59 
food  taking,  64,  68,  69 
imitation  of  activities,  67,  68 
locomotion,  65-67 
phototaxis,  70 

reactions  to  contact,  33-35, 69, 
70 


Amoeba,  thermotaxis,  58 
Amphibia,  intelligence,  226-229 
Amphipods,    phototaxis    changed 
by  contact,  50,  51 
thigmotaxis,  36.    See  also  Or- 
chestia  and  Talorchestia 
Amphipyra,  phototaxis,  36 

thigmotaxis,  36 
Amphithoe,  behavior,  30,  94 
Anemones.  See  Actinians 
Anemotropsm  in  insects,  38 
Anoplodactylus,  phototaxis,  56 
Antennularia,  geotaxis,  30 
Ants,  chemotaxis,  29 

imitation  in,  209,  210,  254 
influence  of  numbers  on  cour- 
age, 207 
intelligence,  192,  193,  205,  214 
variability  in  cocooning,  132, 
133 
Apes,  intelligence,  260  ff 
Aporus,  homing,  195 
Aquinas,  T.,  pleasure  as  a  deter- 
minant of  action  in  ani- 
mals, 164 
Arenicola,  phototaxis  of  larvae,  52, 

53 
Associative    memory,    5    ff.     See 

also  Intelligence 
Asterias,  habit  formation,  154,  155, 

158 
Atherina,  intelligence,  223,  224 

Baboons,  us»  of  weapons,  267,  270 
Bacterium  termo,  chemotaxis,  22 
Bain,  A.,  178,  pleasure  and  pain  in 

accommodation,   173-175 
Balanus,  phototaxis  of  larva,  47 
Balbiani,  E.  G.,  capture  of  food  in 

Didinium,  78 


284 


INDEX 


Baldwin,  J.  M.,  138, 178,  imitation, 
257 
contra  Lamarckism,  124 
organic  selection,  136  S 
pleasure  and  pain  in  accommo- 
dation, 173-175 
problem  of  learning,  172 
Ball,  W.  P.,  138 
Bancroft,  F.  W.,  61,  electrotaxis  in 

Paramoecium,  60 
Barotaxis  defined,  34 
Bateson,  W.,  230,  intelligence  in 

fishes,  220 
Bees,  communication,  215 
homing,  193,  196  ff 
influence  of  numbers  on  pug- 
nacity, 207-209 
intelligence,  193, 196-201, 207- 

209,  215 
and  Lamarkism,  126  ff 
remembering  flowers,  200-202 
Beetles,  intelligence,  192,  206 
Belt,  T.,  intelligence  of  Cebus,  267 

intelligence  of  wasp,  195 
Bentley,  I.  M.,  intelligence  of  chub, 

222 
Bentley,  M.  See  Day,  87 
Berry,  C.  S.,  258,  imitation  in  cats, 
256 
imitation  in  white  rat,   255, 
256 
Bethe,  A.,  16,  189,  215,  conscious- 
ness in  animals,  4,  6 
homing  of  ants  and  bees,  193, 

199 
homing  of  limpets,  187 
lack  of  learning  in  Carcinus, 
182,  183 
Binet,  A.,  89,  behavior  of  Protozoa, 
63-65 
capture  of  food  in  Didinium, 

78 
relation  of  reason  to  percep- 
tion, 248 
Birds,  fear  in,  96,  121-123 
imitation  in,  254,  257 


Birds,  intelligence,  165,  166,  17&- 
178 
migration,  108 

variations  in  instinct,  133.  See 
also  particular  species 
Blanchard,     E.,     intelligence     of 

beetle,  206 
Blow-flies,  instincts,  93 

phototaxis    of  larvae,  21,   42, 
43 
Bodo,  capture  of  prey,  64,  65 
Bohn,  G.,  10,  61,  162,  conscious- 
ness in  animals,  4 
definition  of  instinct,  92 
differential  sensibility,  152 
fatigue  sensorielle,  143 
geotaxis  in  Convoluta,  31,  156 
homing  of  limpet,  118 
law  of  varied  responsiveness, 

145 
phototaxis  of  Littorina,   156, 
157;  of  Pleurosigma,  156 
reactions  of  Cerianthus,   144, 

145 
reactions  of  animals  to  colored 

lights,  47 
rhythms  of  activity,  155  ff 
tidal  rhythms   of   Convoluta, 
31,   156;  of  Pleurosigma, 
156;    of    anemones,    158; 
of  Littorina,  156,  157 
tropisms,  18 
Bostok,  F.,  258 

Branchellion,  interference  of  reac- 
tions, 140 
Brehm,  A.,  279 
Bumble-bees,  208,  209,  215 
Burroughs,  J.,  258 
Buttel-Reepen,  H.  von,  216,  com- 
munication in  bees,  215 
courage  in  bees,  207 
homing  in  bees,  193,  197 
evolution  of  the 
bee  community,  128 

Carcinus,  intelligence,  182-184 


INDEX 


285 


Cardium,  reactions  to  shadows,  41 
Carr,  H.,  and  Watson,  J.  B.,  258 
Casteel,  D.  B.,  231,  intelligence  of 

Chry semis,  230 
Cat,  imitation,  256 

intelligence,  166,  236-242,  255 
Catfish,  intelligence,  222 
Cebus  monkey,   intelligence,   262, 
264-270 
imitation,  270 
Centipedes,   males  eating  eggs  of 

female,  96 
Cerceris,  homing,  195 
Cerianthus,  geotaxis,  30 

reactions  to  repeated  stimuli, 
144,  145 
Cesaresco,  E.  M.,  258 
Chemotaxis,  21-29,  70,  75,  76,  187 
Chick,  imitation,  254 

intelligence,  165,  166,  176-178 
transitory  instincts,  99 
Chilomonas,  chemotaxis,  27 
Chlamydomonas,  geotaxis,  29 
Chromulina,  geotaxis,  29 

phototaxis   affected   by  tem- 
perature, 49 
Chrysemis,  intelligence,  230 
Chimpanzee,  imitation,  272,  273 
intelligence,  262-266,  270,  272, 
273 
Gadophora,     attracting    bacteria, 

22 
Claparede,    E.,    10,    comparative 

psychology,  1,  7 
Gaypole,  Prof.,  imitation  in  young 

ostrich,  254 
Clepsine,  phototaxis,  44 

reaction  to  shadows,  41,  42 
varied  behavior,  111,  142 
Cockroaches,  geotaxis,  32 
Cole,  L,  J.,  61,  phototaxis  of  pyc- 
nogonids,  56 
phototaxis  and  image  forming 
power  of  eyes,  56,  57 
Cole,  L.   W.,   258,  intelligence  of 
raccoons,  242-245,  255 


Communication  in  ants,  213,  214 

in  bees,  215 
Compensatory  motions,  39  ff 

in  relation  to  phototaxis,  57 
Condillac,  instinct,  92 

intelligence  in  animals,  232 
Consciousness,  criterion  of,  3  ff 
Convoluta,  tidal  rhythms  in,  31, 

156 
Cope,  E.  D.,  279,  intelligence  of  a 

Cebus,  266 
Cometz,  v.,  216,  homing  of  ants, 

193 
Corymorpha,  geotaxis,  31 
Cowles,  R.  P.,  learning  in  Ocypoda, 

184 
Crayfish,    compensatory   motions, 
40 
devouring     their     eggs     and 

young,  95,  96 
instincts  as  related  to  reflexes, 
100  ff 

intelligence,  184-186 
reflexes,  13 
Ctenophores,  geotaxis,  30 

statocysts,  30,  33 
Cucumaria,  geotaxis,  31 
Cunningham,  J.  T.,  138 
Cuvier,    F.,    279,    intelligence    of 

orang,  267 
Cypridopsis,    phototaxis    changed 

by  contact,  50 
Cypris,    phototaxis    changed    by 
contact,  50 

Daphnia,  movements  of  eye,  57 
phototaxis   changed   by   con- 
tact, 50 
Darwin,  C.  R.,  114,  138,  162,  279 
chimpanzee   throwing  stones, 

269 
definition  of  instinct,  91 
evolution  of  instinct,  115, 130  ff 
fear  in  young  ducks,  123 

in  young  rabbits,  123 
followers  of,  1 


286 


INDEX 


Darwin,  C.  R.,  imperfect  instincts 

of  Molothrus,  96 
intelligence  of  earthworms,  161 
Lamarokian  theory  of  instinct 

inadequate,  126 
mental  evolution,  260 
reactions  of  earthworms,  28, 

140 
Davenport,  C.  B.,  61 
Davis,  H.  B.,  258,  intelligence  of 

raccoons,  243,  255 
Day,    L.    M.    and    Bentley,    M., 

learning  in  Paramoecium, 

87-89 
Death-feigning.  SeeFeigningDeath. 
Delage,   Y.,   equilibrium  in  Crus- 
tacea, 31 
Delboeuf,   J.,   231,   intelligence  ni 

lizards,  229 
Dellinger,  O.  P.,  89,  locomotion  in 

Amoeba,  66 
Descartes,    R.,    consciousness    in 

animals,  4 
Didinium,    capture    of    food,    64, 

77-79 
Dog,  imitation  in,  256 
instinct  to  swim,  97 
intelligence,  235-237,  240,  245, 

246,  249-251,  255 
Dragon     fly,     larval     and     adult 

instincts  compared,  98 
Driesch,  H.,  geotaxis  in  Sertulari- 

ella,  30 
Drzewina,  A.,  189,  tidal  rhythms 

in   phototaxis   of  hermit 

crabs,  158 
learning  in  Pachygrapsus,  187 

Earthworms,    phototaxis,    21,    43, 
44,  54 
intelligence,  161 
interference  of  reactions,  140 
reactions  to  chemicals,  28 
varied  reactions,  148-,  149 

Earwigs,  thigmotaxis,  36 

Edinger,  L.,  231 


Eimer,    O.    E.,    38,    definition    of 
instinct,  92 

origin  of  instinct,  118,  119 
Electrotaxis,  59  fit 
Eledone,  intelligence,  189 
Emery,  C,  216 

Englemann,  T.  W.,  chemotaxis  of 
bacteria,  22 

influence  of  oxygen  on  photo- 
taxis, 49 
Epeira,  instincts  of  young,  98 
Escherich,  K.,  216 
Eubranchipus,  photataxis,  54 
Eudendriumplanulse,  phototaxis,52 
Euglena,  geotaxis,  29 

phototaxis,  28,  44,  45 

Fabre,  J.   H.,   216,   constancy  of 
instinct,  132 
intelUgence  of  beetles,  206 
locality  sense  of  mason  bee,  197 
nature  of  instinct,  91 
Fatigue    as    cause    of  •  change    of 
behavior,  84, 142-145, 149 
Fear  of  hawks,  not  innate,  123 
in  fiddler  crabs,  140 
in  Rhea,  122,  123 
in  sparrows,  122 
in  young  birds  in  general,  96, 

121-123 
in  young  ducks,  123 
in  young  rabbits,  123 
in  young  terns,  96,  97 
Feigning  death,  change  of  attitude 
in  through  fatigue,  143 
in  young  terns,  96,97 
Fiddler  crabs,  interference  of  reac- 
tions, 140 
phototaxis,  48,  56,  140,  144 
Fishes,  intelligence,  219  ff 
rheotaxis,  37,  38,  57 
Fleure,  H.  J.,  and  Walton,  C.  L., 
162,  reactions  of  Actinia, 
159,  160 
Fly,  behavior  of  decapitated  speci- 
mens, 104 


INDEX 


287 


Fly,  larvae,  See  Blow-fly. 

Forel,  A.,  216 

contra  Lamarckism,  124 
courage  in  ants,  207 
imitation  in  ants,  210 
intelligence  of  bees,  200-202 
intelligence  of  water  beetles, 
192 

Formica  sanguinea,  205,  207,  210 

Fox,  Rev.  D.,  variant  instinct  in 
dog,  134 

Freund,  F.,  258 

Frog,  instincts,  105-107 
intelligence,  227-229 
reflexes,  12-15,  105-107 

Fundulus,  rheotaxis,  37 

Gamer,  R.  L.,  279 

language  of  apes,  214 
Gatke,  migration  of  birds,  254 
Geotaxis,  29,  34 

Verwom's  theory  of,  34 
Ghinst,  van  der,  162 
Gibbs,  D.  and  Dellinger,  O.  P.,  89 
Glaser,    O.   C,    162,    behavior   of 

ophiurans,  181 
Goby,  intelligence,  224-226 
Goldfish,    learning    to    come    for 

food,  223 
Goldsmith,  M.,  231,  intelligence  of 

of  goby,  224-226 
Goltz,     F.,     clasping    instinct    of 

brainless  male  frog,  106 
Gonionemus,  effect  of  hunger  on 

behavior,  150 
geotaxis,  30 
reactions,  16 
Goniotaxis,  defined,  36 
Groom,  T.  F.,  and  Loeb,  J.,  pho- 

totaxis  of  Balanus  larvae, 

47 
Groos,  K.,  contra  Lamarckism,  124 
Gurley,  R.  R.,  231 

Haake,    AV.,    dancing   instinct   in 
shrew,  134 


Habits  in  anemones,  153,  157,  158 
in  birds,  121-123 
in  hermit  crabs,  158,  186,  187 
in  organs,  159 
Paramoecium,  86-88 
in  starfish,  154,  155,  158 
various  causes  of,  153-159 
Hachet-Souplet,  P.,  258 
Hadley,  P.  B.,  rheotaxis  of  lobst€r, 
38,  57 
phototaxis  of  lobster,  55,  56 
Haggerty,  M.  E.,  279,  imitation  in 

monkeys,  273,  274 
Hanel,  E.,  supposed  intelligence  of 

earthworms,  161 
Hargitt,  C.  W.,  162,  reactions  of 
Hydroides    to    shadows, 
142 
Harper,   E.   H.,   61,   photatxis  in 

Perichaeta,  44 
Hartley,   intelligence   in   animals, 

232 
Hartmann,  R.,  279 
Hartmann,  von,  definition  of  in- 
stinct, 92 
Haseman,   rhythms   in   Littorina, 

157 
Hawks,  fear  of,  123 
Heliotropism,  19.     See  also  Photo- 
taxis. 
Heptagenia,  intelligence,  192 
Hermit  crabs,  intelligence,  186,  187 

tidal  rhythms,  158 
Herrick,  C.  J.,  231,  intelligence  in 

cat-fish,  222 
Hertel,  E.,  reactions  of  Paramoe- 
cium to  ultra  violet  rays, 
75 
Hirschlaff,    L.,    279,    imitation   in 

chimpanzee  Consul,  272 
Hobhouse,  L.  T.,  114, 179,  258,  259 
imitation,  256 
instinct  and  intelligence,  112 
intelligence  of  dog,  254 
intelligence  in  monkeys,  262- 
266,    269-270,    276,   277 


288 


INDEX 


Hobhouse,  L.  T.,  problem  of  learn- 
ing, 176 
reason  in  animals,  249, 276, 277 

Hodge,  C.  F.,  and  Aikins,  H.  A., 
change  of  behavior  in 
Vorticella,  89 

Holmes,  S.  J.,   61,  89,   179,  231; 
behavior  of  Loxophyllum, 
16,  25,  35,  79-81,  141 
instincts  of  Amphithoe,  93,  94; 

of  young  terns,  96,  97 
learning  in  crayfish,  185,  186; 

in  sunfish,  223 
phototaxis  of  blow-fly  larvae, 
42,  43;  of  earthworm,  43, 
44,  54;  of  Eubranchipus, 
54;  of  fiddler  crabs,  48, 
56, 140;  of  Jassa,  49;  of 
Notonecta,  55;  of  Or- 
chestia,  46,  48,  49,  51,  55; 
of  Ranatra,  48,  51,  52,  55, 
56,  140;  of  Talorchestia, 
42,  54 
reactions  of  mosquito  larvae  to 
shadows,  142 

Holt,  E.  B.,  and  Lee,  F.  S.,  61 

Horse,  locality  memory,  246-248 

Hudson,  W.  H.,  114,  fear  in  birds, 
121-123 

Huggins,  G.  E.,  learning  in  cray- 
fish, 184 

Huggins,  Dr.,  variant  instinct  in  a 
dog,  134 

Hutchinson,  W.  N.,  258,  imitation 
in  dogs,  256 

Huxley,  T.  H.,  10 

Hyalella,  change  of  phototaxis  by 
chemicals,  50 

Hydra,  geotaxis,  30 

reactions  to  chemicals,  28 
reactions  to  repeated  stimula- 
tion, 141,  145 
thigmotaxis,  36 

Hydroides,  reaction  to  repeated 
stimulation  by  shadows, 
142 


Imitation  in  ants,  209,  210,  254 

in  birds,  254,  257 

in  cats,  255 

in  fishes,  253 

in  monkeys,  262,  270  S 

in  raccoons,  255 

in  rats,  255 

in  dogs,  255,  256 
Insects,  compensatory  motions,  40 

geotaxis,  32 

instincts,  93,  95,  98,  104,  105, 
108,  110,  118-120,  124- 
133,  140,  191  S 

intelligence  in,  191  fif 

photokinesis,  42 

phototaxis,  42,  43,  48,  51,  55 

rheotaxis,  38 

seat  of  instincts,  104 

thigmotaxis,  36 
Instinct,  108 

analysis  of,  21 

deferred,  96,  97 

defined,  92 

dependence  on  internal  con- 
ditions, 110  ff 

evolution  of,  115  fiE 

human,  100 

illustrations  of,  93,  95 

imperfection  of,  95,  96 

and  intelligence,  112 

internally  initiated,  107  ff 

modified  by  experience,  166 

popular  conception  of,  94,  95 

and  reflex  action,  99  ff 

transitoriness  of,  99 

variability,  131  ff 
Intelligence,  beginnings  of,  164  ff 

criterion  of,  181 

deceptive  appearances  of,  159 

in  amphibians,  227-229 

in  apes,  260 

in  birds  (chick),  165,  166,  176- 
178 

in  crustaceans,  182-187 

in  fishes,  219-227 

in  insects,  191  ff 


INDEX 


289 


Intelligence     in     lower     inverte- 
brates, 63  S,  86  ff,  182 

in  mammals,  166,  167,  232  fF 

in  mollusca,  187-189 

in  monkeys,  260 

in  reptiles,  230 
Isopods,  thigmotaxis,  36 

Jackson,    H.   H.  T.,  phototaxis  of 

Hyalella,  50 
James,     W.,     114,     contradictory 
instincts,  112 

instinct  as  determinant  of 
action,  113 

instinct  and  intelligence,  168 
Jassa,  change  of  phototaxis,  49 
Jelly-fish,  reflexes,  13 
Jennings,  H.  S.,  34,  61,  89,  163; 
Aiptasia,  food  taking,  151 

Amoeba,  food  taking,  59,  68; 
locomotion,  66;  role  of 
surface  tension  in  move- 
ments, 67-69 

anemones,  habits,  153;  varied 
reactions,  146,  147 

Chilomonas,  chemotaxis,  27 

Didinium,  capture  of  food, 
78 

earthworms,  varied  reactions 
of,  148,  149 

Euglena,  phototaxis,  44,  45 

Paramoecium,  chemotaxis,  23- 
26;  motor  reflex  in,  73,  74; 
thigmotaxis,  35;  varia- 
tions in  behavior,  76,  77 

Spirillum,  chemotaxis,  27 

starfish  habits,  154-155,  158- 
159,  181,  182 

Stentor,  modifiabiHty  of  be- 
havior, 83 

tropisms,  21 
Jesse,  E.,  intelligence  in  alligator, 
230 

Keeble,  F.,  163,  tidal  rhythms  in 
Convoluta,  156 


Kinnaman,  A.  J.,  279,  imitation  in 

Rhesus  monkeys,  272 
Kirby,  W.  and  Spence,   W.,  216, 

definition  of  instinct,  91 
intelligence  of  beetle,  206 
Knauer,  F.  K.,  intelligence  of  frogs, 

228 
Kollmann,  J.,  189,  intelligence  of 

octopus,  189 
Kreidl,   A.,   32,    fishes  coming  to 

sound  of  bell,  224 
functions     of     statocysts     in 

Palaemon,  31,  32 

Labidocera,  geotaxis,  31 

phototaxis   changed   by   con- 
tact, 50 

Lacordaire,  J.  T.,  intelligence  of 
instincts,  191 

Lady-beetles,  geotaxis,  32 

Lamarck,     J.     B.,     evolution    of 
instinct,  115,  118  ff 

Lamar kian  theory  of  instinct,  115- 
131,  136-138 

Lambs,  undefined  instinct  of  young, 
96 

Landois,  H.,  231 

Lasius,  intelligence,  212,  213 

variability    of    cocooning   in- 
stinct, 132 

Lecaillon,  A.,  231 

Lee,  F.  S.,  61 

Leibnitz,   intelligence  of  animals, 
232 

Leucocytes,  chemotaxis,  26 

Lewes,    G.    H.,     138,    origin    of 
instinct,  116 

LiUie,  R.  S.,  phototaxis  of  Areni- 
cola,  52,  53 

Limpets,  homing,  187,  188 

Limulus,  segmental  reflexes,  16 

Littorina,     tidal     rhythms,     156, 
157 

Lizards,  intelligence,  229 

Lobster,  phototaxis,  55,  56 
rheotaxis,  38,  57 


290 


INDEX 


Loeb,  J.,  10,  17,  31,  61,  conscious- 
ness in  animals,  4,  6 

Amphipyra,  tropisms  of,  36 

geotaxis  in  Antennularia,  30; 
in  Cerianthus,  30;  in 
insects,  32 

Gonionemus,  rhythmic  con- 
tractions of,  16 

heliotropism,  19,  41,  46 

photokinesis  and  phototaxis, 
41 

phototaxis,  change  of  in  cope- 
pods,  Gammarus  and  Pol- 
ygordius,  49 

stereotropism,  33 

theory  of  tropisms,  18-21,  60 
Loxocephalus,  forming  groups,  24 
Loxophyllum,  behavior,  16,  25,  35, 
79-81,  141 

chemotaxis,  25 

reaction  of  pieces,  80 

reactions  to  repeated  stimula- 
tion, 141 

rhythms,  16 

thigmotaxis,  35 
Lubbock,  Sir  J.,  216,  communica- 
tion in  ants,  214;  in  bees, 
215 

homing  of  bees,  193 

intelligence  of  ants,  211-214 

intelligence  of  wasp,  192 
Lukas,  F.,  17 

Lyon,   E.   P.,    compensatory   mo- 
tions, 40 

phototaxis  in  Palaemonetes, 
49,  56 

rheotaxis  in  fishes,  37,  38,  57 

Mactra,  reaction  to  shadows,  41 
Magpie,  imitation,  257 
Mammals,  intelligence,  232  flf 

compensatory  motions,  39 
Man,  instincts  of,  100 
Marshall,  H.  R.,  10,  179 
Massart,  J.,  chemotaxis  absent  in 
Polytoma,  25 


Massart,  J.,  effect  of  temperature 

on   phototaxis  of  Chrom- 

ulina,  49 
geotaxis  in  lower  organisms,  29 
Mast,    S.    O.,    61,    89;    Arenicola 

larvoe,  phototaxis  of,  53 

Didinium,  behavior  of,  77-79; 

Eudendrium,   phototaxis,   52; 

Stentor,  phototaxis,  44 

Maxwell,  S.,  thigmotaxis  of  Nereis, 

36 
May-fly.    See  Heptagenia. 
McCook,  H.  C,  216 
Mendelssohn,  M.,  61 
Metalnikow,    S.,    food    taking    in 

Paramcecium,  72 
Metridium,  food  taking,  151 

varied  reactions  of,  160,  161 
Michener,  E.  J.,  effect  of  chemicals 

on  phototaxis,  50 
Migration,  in  birds,  108 
Mills,  T.  W.,  258 
Milne-Edwards,  H.,  intelligence  of 

animals,  260 
Minkiewicz,  C.,  61,  disguisement  of 

spider  crabs,  104 
reactions  to  colored  lights,  47 
Mobius,  K.,  231,  279,  intelligence 

of  pike,  219,  220 
intelligence  of  a  chimpanzee, 

266 
Mocking-bird,  imitation,  257 
Moggridge,  J.  T.,  on  young  trap 

door  spiders,  98 
Molluscs,  change  of  responsiveness 

to  light  and  shadows,  143 
intelligence,  187-189 
Molothrus,     imperfect    egg-laying 

instinct,  96 
Monkeys,  260  ff 
Montgomery,    T.    H.,    on    young 

Epeiras,  98 
Moore,  A.,  geotaxis  in  Paramcecium, 

30 
Moore,  A.  R.,   habit  formation  in 
starfish,  154,  155 


INDEX 


291 


Morgan,  C.  Lloyd,  10, 114, 138, 190, 
animal  intelligence,  248 
behavior  of  protozoa,  63 
consciousness  in  animals,  4 
deferred  instincts,  96 
evolution  of  instinct,  121 
fear  of  hawks  not  innate,  123 
homing  of  limpets,  187-189 
imitation  in  chicks,  254 
intelligence  in  chicks,  165;  in 
dog,  234,  235;  in  stickle- 
back, 226 
instinct  and   internal  impul- 
sion, 108,  109 
intelligence  in  solitary  wasp, 

203,  204 
Lamarckism  rejected,  124 
organic  selection,  136 
principle  of,  159,  234 
varied   behavior  of  Clepsine, 
111,  112 
Morgan's  law,  159,  234 
Morse,     M.,     tidal     rhjrthms     of 

Littorina,  157 
Mosquitoes,  geotaxis,  32 

reaction  of  larvae  to  shadows, 
142 
Moth,  tropisms,  18 
Motor  reflex,  in  Euglena,  28,  44,  45 
in  Loxophyllum,  79,  80 
in  Paramoecium,  15,  73,  74,  76 
in  Stentor,  44,  81,  82 
Mound-building   bird,   instinct   of 

flight,  97 
Murbach,    L.,    geotaxis    in    Gon- 

ionemus,  30 
Mysis,  geotaxis,  31 

Nagel,  W.  A.,  61,  food  taking  in 
anemones,  150,  151 
light  reactions  of  molluscs,  41 

Natural  selection,  origin  of  instinct 
by,  134  ff 

Necturus,  intelligence,  227 

Nephelis,  phototaxis,  44 

Nereis,  thigmotaxis,  36 


Nervous  system,  of  crayfish,  101 
Nightingale,    variation  in  singing 

instinct,  133 
Norman,  W.  W.,  pain  sensations  in 

lower  animals,  8 
Notonecta,  phototaxis,  55 
Nuel,  J.  P.,  61 

Octopus,  intelligence,  188,  189 
Ocypoda,  intelligence,  184 
Odynerus,  instinct,  118 
Oelzelt-Newin,  A.,  90,  food  taking 

in  Loxophyllum,  80 
Orang,  intelligence,  267 
Orbitolites,  thigmotaxis,  34 
Orchestia,  phototaxis,  46,  48,  49, 

51,  55 
Organic  selection,  136-138 
Osborn,  H.  F.,  organic  selection, 

136 
Osmia,  parental  instincts,  120 
Ostrich,  imitation  in  yoimg,  254 
Oxytricha,  forming  of  groups,  24 
thigmotaxis,  35 

Pachygrapsus,  intelligence,  187 
Pagurus,  intelligence,  186 
Pain,  6,  164  ff 
Palsemon,  32 

statocysts,  31  ff 
Palaemonetes,    change    of     photo- 
taxis, 49 
Paley,  W.,  conception  of  instinct, 
108 

definition  of  instinct,  91 
Paramoecium,  behavior,  15,  35,  71 

chemotaxis,     21,    23-25,     27, 
75,  76 

choice  of  food,  72 

effect  of  hunger  on,  150 

electrotaxis,  59,  60 

forming  groups,  24 

geotaxis,  29,  30 

interference  of  reactions,  140 

modifications  of  behavior,  76, 
77,  140 


292 


INDEX 


Paramoecium,  motor  reflex,  15,  73, 
74,76 
reaction  to  ultra  violet  rays, 
75 

swimming,  72 
thermotaxis,  58,  75 
thigmotaxis,  35,  73,  76 
Parker,    G.    H.,    food    taking    in 
anemones,  150,  151 
geotaxis  of  Labidocera,  31 
phototaxis  of  Labidocera,  31, 
50;  of  Vanessa,  55 
Parrot  imitation,  257 
Patellas,  homing,  188 
Pawlow,  J.  P.,  formation  of  habita 

by  stomach,  159 
Peal,  imitation  in  jungle  pheasant, 

254 
Pearl,     R.,     163,     goniotaxis     in 
Planaria,  36 
reactions  of  Planaria,  53,  147 
variability     in     behavior     of 
Planaria,  147 
Pechuel-Losche,  E.,  intelligence  of 

baboons,  267,  269 
Peckham,  G.  W.  and  E.  G.,  216, 
homing  of  wasps,  193-196 
instincts  of  solitary  wasp,  119, 

120,  131,  132 
intelligence   in  solitary  wasp, 

203,  204 
variability  of  instinct  in  soli- 
tary wasp,  131,  132 
Perch,  inteUigence,  220,  221 
Pfeffer,  W.,  chemotaxis  of  bacteria, 
22 
chemotaxis  of  spermatozoida 
of  ferns  and  mosses,  26 
Pfungst,  O.,  258 
Phototaxis,  19-21,  36,  38,  40  ff,  82, 

186,  192 
Photokinesis,  41,  42 
Pieron,    H.,  intelligence   of  Cras- 
sius,  221 
intelligence  of  Triton,  227 
kinaesthetic  reflex,  199 


Pigeon,  compensatory  motions,  39 
Pike,  intelligence,  219 
Planaria,   reactions  to  chemicals, 
28,53 
reactions  to  light,  41 
reactions  to  mechanical  stimu- 
lation, 53,  141 
reactions  to  repeated  stimula- 
tion, 141 
thigmotaxis,  36 
varied    response    to    stimuli, 
144,  147 
Plants,  reactions,  15 
Play  instinct,  109 
Pleasure,  6,  55,  164  ff 
Pleurosigma,  tidal  rhythms,  156 
PoUstes,  intelligence,  195,  196 
Polyergus,  stupidity,  210,  211 

variation  in  cocoon  spinning, 
132,  133 
Polytoma,  lack  of  chemotaxis,  25 
Pouchet,  G.,  phototaxis  of  Temora, 

50 
Preyer,  W.,  163,  behavior  of  star- 
fish, 181 
human  instincts,  100 

Promethus  moth,  instincts  of,  124, 

125 
Protozoa,  internally  initiated  activ- 
ity, 109 
intelligence,  63-65,  86  ff,  181 
reactions,  15,  21  ff,  44-47,  49, 
68-60,  63  ff,  140, 141,  150 
rhythms,  16 

tropisms,  18,  21  ff,  44-47,  49, 
5S-60,  70  ff 
Prowazek,  S.  von,  90 
Psammobia,  reaction  to  light,  41 
Putter,  A.,  61,  90 
Pycnogonids,  phototaxis,  56 

Raccoon,  intelligence,  242-245,  255 
Rddl,  E.,  62,  hovering  in  flies,  38 
movement  of  eye  of  Daphnia, 
67 


INDEX 


293 


R^I,    E.,    phototaxis  of   Arthro- 
poda,  55 
relation  of  phototaxis  to  vis- 
ion, 56 
Rae,  Dr.,  fear  in  young  ducks,  123 
Ranatra,  behavior  after  decapita- 
tion, 105 
Interference   of   reactions,    5, 

140 
phototaxis,  48,  51,  52,  55,  56, 
144 
Random   movements,    in   chemo- 
taxis,  24-26 
in  phototaxis,  21,  22,  41-45 
r61e  of  in  learning,  168 
Rat,  imitation,  255,  256 
Raven,  imitation,  257 
Reason,    in    apes    and    monkeys, 
274  ff 
in  insects,  203  flf 
in  mammals,  232  ff 
Reflex  action,  11  ff 
Reflexes,  2,  11  ff 

of  pecking,  swallowing,  176- 
177.      See      also     Motor 
Reflex, 
Reflex  action  and  instinct,  99  ff, 

116,  117 
Reighard,  J.,  231,  intelligence  of 

fishes,  223,  224 
Reimarus,  H.  S.,  114 
Reptiles,  intelligence,  229,  230 
Reuter,  O.  M.,  intelligence  of  ants, 

193 
Rhea,  no  innate  fear  of  man,  122 
Rheotaxis,  34,  37  ff,  57 
Rhesus  monkey,  intelligence,  262-      Shepherd,   W.,   279,   imitation  in 


Romanes,  G.  J.,  10,  114,  138,  279, 
animal  inteUigence,  2 
definition  of  instinct,  91,  92 
evolution     of    instinct,    120, 

121 
homing  of  bees,  196 
intelligence  of  dog,  250 
moth  flying  to  candle,  20 
Romanes,     Miss,     intelligence    of 
Cebus,  265,  269,  278,  279 

Sachs,  H.,  tropisms,  19 
Sagartia,  effect  of  hunger  on  food 
taking,  150 
geotaxis,  30 
Schaeffer,  A.  A.,  90,  231,  food  tak- 
ing in  Paramoecium,  72 
selection  of  food  in  Stentor,  85, 

86 
intelligence  of  frog,  228,  229 
Schneider,    G.    H*   114,    190,   in- 
stinct, 1 
intelligence  of  octopus,    188, 
189 
Schneider,  K.  C,  114 
Schrader,    M.,    17,    segmental   re- 
flexes in  frog,  107 
Sense  of  direction,  in  ants,  193 
in  bees,  193,  196-202 
in  horses,  247 
in  wasps,   193-196,   200, 
201 
Serpula,    reactions    to    light    and 
shadows,  41 
varied   reaction   to   shadows, 
145 


265 
imitation,  271-274 
Rhumbler,    L.,    90,    behavior    of 

Amoeba,  68 
Rhythms,  in  behavior,  16,  79,  80, 

155-158 
Righting    movements,    in    Loxo- 
phyllum,  81 
in  starfish,  154,  155,  158 


Rhesus  monkeys,  272,  273 
Sherrington,  C.  S.,  17,  reflex  action, 

11 
Small,   W.   S.,   258,   imitation  in 

white  rat,  255 
Smith,     S.,     90,     educabiUty     of 

Paramoecium,  86-89 
Sokolowsky,  A.,  279 
Solen,  reaction  to  shadows,  41 


294 


INDEX 


Sondheim,  M.,  216,  intelligence  of 

damsel-fly,     larvae,    191, 

192 

Spallanzani,  Abbe,  clasping  instinct 

of  brainless  male  frog,  106 

Sparrows,  fear  in,  122 

Spaulding,  D.  A.,  114,  instincts  of 

chicks,  99 
Spaulding,  E.  G.,  190,  learning  in 

hermit  crabs,  186,  187 
Spence,  W.  See  Kirby. 
Spencer,    H.,    10,    114,    138,    179, 
definition  of  instinct,  91, 
107 
Lamarckian  theory  of  instinct, 
127 

mental  evolution,  1,  9 
origin  of  instinct,  116,  117 
pleasure  and  pain,  167,  172- 

175 
relation  of  intelligence  to  other 
activities,  139 
Spider  crabs,  instincts  for  disguise- 

ment,  104 
Spiders,  devouring  their  eggs,  96 
geotaxis,  32 
instincts  of  young,  98 
Spirillum,  chemotaxis,  27 

geotaxis,  29 
Spirostomum,  geotaxis,  30 
Stahl,  E.,  chemotaxis  in  myxomy- 

cetes,  23 
Staphylococcus,     attracting     leu- 
cocytes, 26 
Starfish,  habit  formation,  154-155, 

158 
Statkewitsch,  P.,  reaction  of  Para- 
moecium     to     induction 
shocks,  144 
Statocysts,  functions  of  in  Crusta- 
cea, 31 
function  in  Ctenophores,  30 
Stentor,  effect  of  hunger  on,  150 
modifiability  of  behavior,  49, 

82-86,  141,  145,  150 
phototaxis,  44,  49,  82 


Stentor,  reactions,  15,  81-86 

rhythms,  16 
Stereotropism,  33  ff,  42 
Stickleback,  intelligence,  226 
Stoichactis,   varied  reactions,   146 
Stone,  intelHgence  of  dog,  250,  251 
Strasburger,  E.,  62,  effect  of  tem- 
perature on  phototaxis  of 
swarm  spores,  49 
Sunfish,  intelligence,  223 
Surface  tension   theory  of  move- 
ments of  Amoeba,  67 
Swallows,  instinct  of  flight,  97 
Swarm  spores,    change  of  photo- 
taxis, 47 

Talorchestia,  photokinesis,  42 

phototaxis,  42,  54 
Temora,    phototaxis    changed    by 

contact,  50 
Temperature,  effect  on  phototaxis, 
49 
reactions  of  organisms  to,  58, 
59 
Terns,  fear  in,  96,  97 

feigning  death,  96,  97 
undefined  instinct  of  young,  96 
Thermotaxis,  58,  59 

in  ParamcEcium,  58,  75 
in  Planaria,  59 
Thigmotaxis,  33-36,  42 
Thompson-Seton,  E.,  258 
Thomdike,  E.  L.,  231,  258,  279, 
animal  intelligence,  nature 
of,  235-242,  251,  252,  255 
imitation  in  monkeys,  270 
intelligence  in  cats,  166,  167 
intelligence  of  monkeys,  261, 
262 
Titchner,  E.  B.,  10 
Toads,  intelUgence,  227,  228 
Toenjies,  H.,  258 
Torelle,  E.,  62 

Torrey,  H.  B.,  effect  of  hunger  on 
food  reactions  of  Sagartia, 
160 


INDEX 


295 


Torrey,  H.  B.,  geoUxis  in  Cory- 

morpha,  31 
geotaxis  in  Sagartia,  30 
Tortoise,  intelligence,  229,  230 
Towle,    E.,    phototaxis   of   Cypri- 

dopsis,  50 
Triplett,   N.,   231,   intelligence  of 

perch,  220,  221 
Triton,  intelligence,  227 
Tropisms,  2,  18  ff 
Turner,    C.    H.,    216,    homing    of 

burrowing  bees,  198,  199 

homing  of  mud  dauber,  200 

Twain,  Mark,  imbecility  of  ants,  95 

Uexkull,  J.  von,  17, 163, 190,  intelU- 
gence  in  Eledone,  189 
tonus,  149 

Verwom,  M.,  62,  90,  function  of 
statocyst  in  Ctenophores, 
30 

geotaxis,  34 

thigmotaxis,  34 

thigmotaxis  in  Oxytricha,  35 
Voltaire,  91 
Volvox,  phototaxis,  47 
Vorticella,  change  of  behavior,  89 

reactions,  15 

rhythms,  16 

Wagner,   G.,  reactions  of  Hydra, 

141,  145 
Wagner,  M.,  217,  bumblebees,  208, 

209,  215 
communication  in   bees,    215 
homing  of  bees,  193 
WaUace,  A.  R.,  280 
Walter,    H.   E.,    62,   reactions   of 

planarians  to  changes  of 

light  intensity,    142-143; 

to  repeated  jarring,  141 
Walton,  C.  L.  See  Fleure,  159. 
Washburn,  M.  F.,  10 
Washburn,  M.  F.,  and  Bentley,  I. 

M.,  231 


Washburn,  M.  F.,  intelligence  of 

chub,  222 
Wasmann,  E.,  10,  114,  217,  com- 
munication in  ants,  214 
courage  in  bees,  207 
imitation  in  ants,  210,  254 
inteUigence  of  animals,  nature 

of,  181 
intelligence  of  ants,  193,  205, 

211 
intelligence  of  apes,  268,  276, 

277 
plasticity  of  behavior,  139 
Wasps,  homing,  193-196,  200,  201 
intelligence,  192-196,  200,  201, 

203,  204 
instincts,  119,  120,  131,  132 
Watkins,  G.  P.,  90 
Watson,  J.  B.,  258,  280,  imitation 

in  monkeys,  270,  272 
Weismann,    A.,    138,    contra    La- 

marckism,  127 
Wheeler,  W.  M.,  217,  imitation  in 

ants,  209,  210 
White,  G.,  intelligence  in  tortoise, 

229,  230 

Whitman,  C.  O.,  138,  evolution  of 

instinct,  115, 116, 121, 135 

Lamarckism,  124 

intelUgence  of  Necturus,  227 

origin  of  instincts  in  pigeons, 

135 
varied  behavior  of  Clepsine,  111 
Willcox,  M.  A.,  190 
Witmer,   L.,   280,   intelligence    of 
chimpanzee,  266,  267 
imitation  in  chimpanzee,  272, 
273 
Wodsedalek,     J.,    intelligence    of 

May-fly  nymph,  192 
Wundt,  W.,  138,  origin  of  instinct, 
118 

Yerkes,  A.  W.,   163,  reactions  of 
Hydroides    to    shadows, 


296  INDEX 

Yerkes,    R.    M.,    190,    231,    258,  Yoakum,  G.  S.,  259 

intelligence   in    Carcinus,  Yung,  E.,  190 
184;  in  crayfish,  184;  in 

frog,  228;  in  turtles,  230;  Zell,  T.,  259 

variability  of  behavior  in  Zeigler,  H.  E.,  114 

dancing  mouse,  133,  134  Zurn,  F.  A.,  259 


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